A "TK" is when a player kills a teammate in multiplayer. (Team Kill)
In Multiplayer
Teamkilling can only be accomplished with friendly fire on during a team-style game. Some servers will strictly prohibit TKing, while others are more lenient. TKing may give you a vehicle or a weapon that the target is operating, or it may cause them to drop an objective, such as an oddball, so you can improve you ranking for that game.