A team player is someone who places the well being of the team ahead of the well being of him-/herself. A team with members who act cooperatively and seek to achive the common goal functions better than a team with members that only focus on their own individual goals.
A team player will sometimes act in a way that might not be optimal for him as individual in the short term (in fact it might even be determental) in order to . This is called 'taking one for the team'.
Sometimes small individual sacrifice can drastically help other team members.
See: Heroes, Game Theory, Selfishness