Short for dragon kill points, DKP is a point system that originated in the online game EverQuest. Since then, it has come to be used in many MMORPGs to help players fairly distribute the loot obtained when a large party of players defeats an enemy. For each player that participates in killing a large creature or other large mob, the player earns points, often based on the difficulty and amount of players participating. DKP are kept track of by the guild leader and used to loot creature drops. The player that has participated the most gets the most points and be entitled to the rare or otherwise valuable loot first.
Each game and guild has their own method of obtaining and keeping track of DKP. When joining a guild in an online game, it is best that you talk with the guild leader or visit their personal page for additional information about how they manage the guild DKP.