A table captain is a player who knows it all and who doesn't mind sharing it with the other players at the table. He'll give a running commentary on play, cite odds and try to direct the action at the table. Table captains have usually read and studied some, and may or may not know how to apply what they know.
Unfortunately, table captains can tighten up a game as gambling players or players who don't study worry about having such a "smart player" at their table. They also let unstudied players know there's more to this game than just cards and luck.