In American sports slang, a bench warmer is a slightly-derogatory term for a reserve player who doesn't get much playing time. The term is derived from the fact that these players sit on the bench so much that it gets warm beneath them, although this is of course hyperbole. Not every reserve player is a "bench warmer"--- depending on the sport and level of play, reserve players can be a regular part of the rotation. The true bench warmers are players that are only called out in case of injury or exhaustion by the other players.
Depending on the level of play, the reason for having bench warmers varies. In recreational or lower-level highschool or college sports, participation is encouraged, even if the player may not be that skilled. Players who are on the team for the social aspects might therefore be stuck in a position where they don't actually play much. On the other hand, in professional sports, where the rewards and resources can be so great, teams have many players because they can afford to prepare for every contingency, such as multiple players being unable to play. Also, it is important to remember that in professional sports, the bench warmers are still elite athletes by almost every standard. Players who might only play for a few minutes a game professionally were probably still stars on very good college teams.