Dark parking is a nickname given by necromancers and shadow knights to the specifc tactic of snaring one mob in a group, leading the mobs away from their spawn points, and then feigning death, causing the snared mob to end up isolated from its friends. (Being snared makes NPCs not path back to their spawn points.)
The word dark refers to the Clinging Darkness line of snare spells that both necromancers and shadow knights possess, while park refers to the fact the mob to be isolated remains "parked" in place once the puller feigns.