One person is the “dungeon master”. They are sort of like a referee. They typically make the initial pitch of the game to the other players. That’s something like “I want to do a game about defending a town from zombies” or “wouldn’t it be cool to be like bank robbers in a fantasy world?”

The group then comes up with characters they want to play. DND has rules for what sort of powers and abilities characters have. This is mostly captured by “character class” (fighter, wizard, paladin, etc). You might say "I want to hit things with a sword so I’ll be a fighter " or “I want to learn magic so I’ll play a wizard.”

Unlike playing a board game, you also come up with some story bits for your character. So you might play a wizard who’s fresh from wizard school and turned to bank robbing to pay off student debt. Or an old wizard that needs money to pay for expensive magical healing. It is pretty wide open, though not totally open. Some new players will try to do like “my dad is the king and my mom is an angel and I can fly and…”, and forget to sync their story with the rules parts. And ideally whatever story idea you have hooks into the pitch.

Once you’ve done all that, you get together as a group (virtually or in person). The DM (dungeon master) will describe the initial scene. “You’re just kicked open the door to the bank office. Your tip said it was going to be empty, but two elves are sitting behind a large desk. They jump up, startled and surprised. The room is ostentatious. Marble floors. Stained glass floor to ceiling windows. Magical lights hanging from the fifteen foot ceiling.” The dm will probably ask what you do.

Note that the dm generally describes the scene and characters that aren’t being played by one of the players. The players in DND generally only control their specific characters. In dnd a player could not be like “the elf runs away in a panic!” or “suddenly all the lights go out!”. Only the dm can declare what the world and its non-player inhabitants do. The players control their characters only. (non DND RPGs may differ on this point)

A player may ask questions. “are the elves armed?”

The dm may answer. “you don’t see any obvious weapons”.

The dm may ask you to roll dice to determine if you succeed at something, notice something, or know something. “Roll perception for me to see if you notice anything else.”

When you check if you succeed at something like that in DND you roll a 20 sided die, and often add something to the result based on your character. A smart character has a bonus for knowing stuff, and a strong one to physical challenges, as examples.

“That’s a 17 on perception? You notice a magical wand on the desk, half covered by papers, and the elf glanced at it.”

Players may declare they’re doing something. “I want to lunge for it before the elf picks it up!”

The dm may call for a roll to see how that goes, and narrate the outcome.

This process of the dm describing the scene and the players asking questions / taking actions continues. If a fight breaks out, combat in DND has a lot of specific rules for determining if attacks hit or miss, if someone is taken out of the fight, and so on.

Usually people play for a few hours at a time, and pick up the story the next time they play. So the first session might be trying to rob the bank vault and getting in a fight with the staff, the second might be cracking the vault and escaping, the third might be a showdown with the head of security with a horseback chase. Much of this will be made up by the DM. They may not have expected the players to steal horses and ride into the desert, but if that’s what the players do they will make something up. This is the big appeal of RPGs over videogames. You can do anything the group thinks is cool. It doesn’t need to be scripted ahead of time.

Much of this as I’ve described it applies to non-dnd RPGs, too. DND has a lot of very specific things about it I’ve kind of glossed over. It has very idiosyncratic combat and magic systems, for example. You’ll hear people talk about “roll for initiative”, “saving throw”, “critical hit” and more. Those are details for dnd’s rules.

Honestly DND is kind of a bad first rpg. It’s very popular but it’s also extremely janky, and only does adventuring day fantasy at all well. Games about political intrigue or social conflict don’t do well in DND specifically because it doesn’t really have detailed rules for them. It’s also pretty harsh on allowing players to be creative compared to other games.

This is very long but I hope it was helpful.

Tabletop Role playing Games are games set in a conversation.
you and your friends talk, telling a story and in doing so, observing the rules et of the chosen game.

This is different from just collaborative storytelling because, when it is doubtful wether an action might succeed, you roll dice to decide. When to roll dice will be part of your chosen set of rules.

Dnd and many other traditional TTRPGs divide roles: one person is a Narrator (dungeon master) while each other person plays one of the protagonists (player characters). There are also games that swap role of Narrator, let you play all by yourself without one or any other constellation. Dnd is a classic game with a Narrator.

Now, if you and your friends decide to play, you will need reference to the rules. With help of these, you can craft the player characters, and the Narrator can think of a compelling story to drop these characters into. You then talk about the fictional situation and then the players controlling the protagonists decide how their characters react to the situations the narrator lays out. When something is easily done, ie opening an unlocked door, you just say that you do it. If something is hard and success is dubious, like rappelling over a ravine, you might be called to roll a dice. While playing dnd, this will typically be a 20 sided dice, and you will typically add some modifiers based on your character.
your result will then tell you wether that action succeeded or failed. This changes the narrative, so you take some time to describe what happens and then move on with your conversation-story.

D&D is a tabletop role-playing game system.

Each of the players is playing a role, you’re basically pretending to be your character.

Then there’s the game master/dungeon master (different systems use different terminology.) Their job is basically to make up the world your characters live in and tell you what’s around you.

Then it’s up to you how you interact with that world.

So your DM makes up a world, let’s say for the purposes of illustrating things it’s a generic high fantasy medieval setting, and the kingdom you all live in is being tormented by a dragon. In the game, the DM will play the role of everyone else in the world, the king, the dragon, all of the townsfolk, etc. The players just play their own character.

For convenience sake, DM will usually start off by bringing the party together somehow. Maybe you’re all drinking in the same tavern when the dragon attacks, maybe the king has summoned you all to the castle because he’s putting together a team to slay the dragon, maybe you’re already an established adventuring party and you’re just walking into town after your last adventure.

And from there, what the players do is their own choice. Maybe you agree to go hunt down the dragon, either because it’s the right thing to do, or because you want the reward money. Maybe you decide to ignore the dragon plot entirely, hop on a ship and start a new life in another kingdom (although nothing is stopping the DM from saying that all the ships were destroyed in the last dragon attack, or the dragon attacks your ship as you’re leaving port)

Every character, whether they’re a player character or one the DM controls has a list of stats- strength, dexterity, charisma, intelligence, etc. That represent how good they are at different things. A fighter is usually going to be strongstrong, a wizard is smart, a bard is charismatic, etc.

To do an action, you roll dice to see if you succeed. You also add or subtract to that dice roll based on what your stats are. So if you’re trying to break down a door, you would add your strength modifier to your roll. So say you roll a 10, if you’re a strong character you would probably add on to that, making it an 11 or even higher depending on exactly how strong you are. If your character is very week, you may actually subtract from your roll. Then your DM tells you if your roll with the modifiers added/subtracted was high enough to break down the door.

Same basic idea goes for making an attack against an oponent, trying to be stealthy, just looking around to see if you notice anything, trying to lie to someone, doing first aid after you or a party member has been hurt, etc. Roll dice, add modifiers, and see if it beats whatever number the DM decides it needs to beat.

And using those mechanics you go off on an adventure.

That’s of course very simplified, but hopefully that gives you the basic idea.

You may often see tabletop RPG players using minis, maps, model buildings and terrain on the table. This is actually optional. You can do it all in your imagination and by talking it out, but a lot of people find it useful when visualizing combat, or navigating a dungeon, etc. to have little physical pieces they can move around. Many groups will do a mix of both, a lot of stuff happens in the “theater of the mind” without any minis or maps, but bust them out when the situation calls for it. All you need is something to write with to make your character sheets, the rulebooks, and some dice.

One player is a storyteller called Dungeon Master (DM) who tells the story for other players. Other players are the adventurers, representing characters with their own skills, personalities and alignments. When the story continues, DM might ask the players how they would like to react to some situation, where to go next or what to say to a non-player character (NPC) that DM controls. When a player tries to do something (like push a stuck door open), DM decides if the player has to make a “check” to see if he succeeds or not. Checks involve throwing dice (d4, d6, d8, d10, d12 or d20 mostly) according to player’s character’s skill on that perticular action, like “strength” in this case. A physically strong character throws bigger/more dice than a weak one in this case. DM pre-decides the sum that the player has to reach with the dice throw, and then the story continues in a different way depending if player succeeded or perhaps failed and triggered a trap, or something. D&D often includes fighting bad guys (or good guys, if you are playing the villains!) which also works with checks and skills, and the players deciding what they are trying to do. Lots of room for improvisation and imagination, which makes the game so fun.