Pk usually stands for Prydwyn Keep (check spelling) in the Camelot hills. Its a good place to bind if you heading to df (darkness falls lol) or the surrounding area. Also, many ppl bind at PKB (Prydwyn Keep bridge) which is nearby.
um.. the PK you are most likely hearing ppl refer to is usually a term from the pvp servers but can be used in all rvr. it means "player-kill" which means you killed/got killed another player.