What does "I'm game"

"I'm game" means that you're willing to try something. In this context, the word "game" means having a willingness to take a risk, to undertake something that could involve effort or injury to the participant. (It's a word that's often used by breeders of pit bull dogs-- a dog is "game" if he's aggressive and undeterred by the displays of aggression that other fighting dogs might display.)