The 2 letters P and K stand for: Player Kill.
PK'er would be Player Killer.
More than one PK'er would be: PK'ers.
Basically a PK'er is a person who character's sole perpose is to kill other people's characters, for whatever reasons. It can happen in almost in game that let's 2 or more characters go head-to head or duel as some may call it.
Comes from the old style short hand typing inside closed realms of the online game Diablo and/or Diablo 2 LoD.