I'm glad you've ntoiced it, ccexplore!
Karma, basically is a bit like a post rating system. If you like a post, you press the up arrow, if you dislike a post / think it's spam / off topic, you press the down arrow.
Hopefully, it'll help keep some members on the straight and narrow.
So, if, say, for instance somebody posted a piece of info you requested, you could thank them by pressing the up arrow, and if someone disrupts the flow of a thread, you can press the down arrow!