The concept is very simple, we are a social network and by giving someone "thumbs up karma" you are letting this person and the community know that there is something good about it.  On the other hand when you give someone "thumbs down karma" usually means that there is something not nice about that person.  Every time you give someone a "thumbs down karma" a good karma point is discounted, if there is no good karma then that person goes in to the horrible minus karma (will appear with the minus sign, example: -5)