A short, simple explanation: it's kinda like a classic fantasy book except you make all of the decisions for one character. The DM isn't quite the same as an author because he mostly controls how the world reacts to the players and the events that happen as you travel, but the players are all essentially the protagonists of the story

Whenever there's a chance for failure, you roll dice to see if you succeed. Some characters are better at certain actions than others are so the game works best when the players cooperate

There's also a tactical aspect and some resource management to the game, but you can be eased into that without issues