The general idea is that for good posts, you give good karma (for saying something insightful or intelligent or whatever), but for bad posts, you give bad karma (for being stupid, or spammy or whatever). Thus, one would be able to tell the difference between those who are valuable members (high karma == good posts) and those who are not (low karma == bad posts).
That said, there's no real way to verify any of this, and if you were dedicated enough, you could easily sky rocket someone's karma (24 points/day), or plunge it into the dankest pits of hell (same rate). Or whatever.
I wonder sometimes if I shouldn't just turn it off...
