From the Introduction to the Dirty-Ass DexCon 13 Edition:
Misspent Youth is a science fiction game about friendship and rebellion. It's a roleplaying game, which means you create a world, pretend to be people you're not, and create a story in real time as you play the game.
The protagonist in the stories you make is called a Youthful Offender (YO), and is a heroic kid between 12 and 17 years old who won't put up with being oppressed. The antagonist is called The Authority, the force that is fucking up your world and making it a shitty place to live. You or one of your friends plays the Authority and everyone else plays a single YO.