It’s a form of make-believe with rules. Basically there’s a guy who narrates the story and makes things up as he goes along called the Dungeon Master. There are players who each have characters that they have created by rolling dice to determine stats and creating their own backstories. The Dungeon Mater sets the scene for the adventure and asks the characters what they want to do. If they fight monsters, then they’ll roll dice to determine if they hit, how much damage they’ve done etc. The game never really has an ending, it just goes on and on until the players either die, or they get bored and quit.