It's a karma system. If someone helped you out with something, then you can give rep as gratitude, or if the person said something funny or something you agree with, you can give rep. "Social points" would be one way of describing it. Now, if someone is acting like a prick, then you can take rep. Karma goes both ways. In a nutshell, it helps to let others know whether you're one of the good guys or just an asshole, but rep power shouldn't be taken seriously. As I said before, they're just social points.