A Baltimore chop is a ball that is hit sharply downward to produce a high bouncing ground ball. An ideal chop will bounce so high that the batter can leg it out for an infield single by the time the ball has come down for a fielder to field it.
The Baltimore chop was named because it was a favorite weapon of the 1890s Baltimore Orioles. The chop is most effective if the batter is able to chop the ball off the plate, but having very hard ground in front of the plate makes a successful chop more likely. The Orioles supposedly had their groundskeeper treat the area in front of home to make it especially hard- just one example of their creative groundskeeping.