Explain Like I'm Five

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.