A DKP is dishonor kill point which gives you a minus rating if you did it when you had 0 so if you get -50 DKP you could effectively receive a demotion in the battle field resulting from this. Losing -50 DKP will get you +50 DKP so how could that be bad.
Or the more familiar term in which people use it could also mean Dragon Kill Points too which is a kind of curancy in endgame raid content and determins who get all the loot etc. Say for example there a real good end game raid and each person who went to that would get a certain amount of points. Then if they wanted an item, you'd get some DKP taken away from you.