It means to 'fix' it in the sense of setting it up so that a certain person will win. If a game is 'rigged' it isn't a fair game.

To 'rig' a game means to bring about certain circumstances within the game, swaying it in a manner which brings the game into disrepute. Basically, the game becomes unfair. It's a weird word though... I don't think I've ever used it in a sentence before *-*