Bunch of people gather around a table. One person is the 'Dungeon Master'. Everyone else are 'Player Characters'. The 'Dungeon Master' makes up a heroic-fantasy scenario ("City under attack by goblin hordes"), and the 'Player Characters' all attempt to resolve it, or frustrate the DM with their antics ("I seduce the goblins"). To determine success or failure of actions, dice are rolled.

From one non-player educated through memes, podcasts, and Baldur's Gate to another: You and the boys try to collaboratively write a book while one sadist and fate fuck it up.

Building on other replies you've gotten, if you're familiar with games like Skyrim, D&D is a bit like that.

Except instead of a computer programme with hard-coded responses for all the NPCs and hard-coded quests and ways for the player to solve quests, a human being called the Dungeon Master plays all the NPCs and adjudicates all your actions. This is the key to tabletop RPGs, because it means you can truly try anything. You're free to really think outside the box. Need to get past a guard into a castle? A computer game might give you the option of stealth or kill. In D&D you might seduce him, put on disguises, bribe him, climb up a window and bypass him, or anything else you can think of. Some videogames might give you that many choices some of the time. But TTRPGs like D&D give you all the choices all the time.

Usually you play with 3–5 players plus a DM.

In terms of how it actually plays at the table, instead of the computer determining if your stealth succeeds, how much damage you do, etc., you roll dice and add numbers based on your character abilities.