From BR Bullpen

The Bench can refer literally to a physical bench located in the dugout on which players and coaches sit while they are not involved in the game, either because they are substitutes, or because the team is at-bat and their turn in the batting order has not yet come.

Metaphorically, the bench refers to the group of position players who are not starters (it does not include pitchers, as these are part of the bullpen). A team can thus be said to have a "good bench", meaning that it has strong substitute players. A bench player is a player who is not normally a starting player, while a "bench warmer" is slang for a player who rarely gets used in a game.