Karma is a sort of user-rating system, that allows people to "give karma" or "deduct karma" other users. What you see under people's forum profiles is the total karma that user has, showing both positive and negative karma.
Karma is basically a way to rate people, their forum attitudes, and their posts and contributions. If you like the person, their uploaded videos and files, and what they are posting you can increase their "karma". If not you can decrease it. It's just a social status meter.
NOTE: Only members with a minimum of 10 posts may add or remove karma points.