a phrase used in the game of baseball to call up a player to take his turn at bat. During a game, members of one team take turns trying to hit a ballpitched (thrown) towards them. The player attempting to hit the ball is called the batter. Up, as used here, means "having a turn; so as to have a turn." The phrase batter up is called out by the umpire (the official who supervises the game and enforces the rules) to indicate that it is time for the batter to move into position to bat the ball so that play can start.