This is a baseball term which describes a type of hitter who typically hits the ball to the same side of the field they hit the ball from. For example, a left-handed player who bats from the left side of the plate, from the first-base side, will typically hit the ball to the same side of the field, which in baseball terms is right field. If the same player were to hit the ball into left field, the player would be considered hitting towards the "opposite field." The reason for left field/right field difference between batter and the field is baseball's use of the viewer's perspective to assign sides.
It is helpful for the defense to know if a player at the plate is a pull hitter and to what degree. If the player always hits to the same side of the field as they bat, they might be known as a "dead pull hitter." However, most players do hit to the opposite field every once in a while. If the defense has a good idea of where the player is likely to hit the ball, the manager can shift their defense to those positions. For the same left-handed batter mentioned above, the manager of the defensive team might move his third baseman and shortstop an entire position to the left, putting the 3rd baseman in the shortstops usual position and so on, while moving the second baseman back and the first baseman to the first base line. This does create an opening on the opposite side of the field, also known as a gap, for the hitter to hit into.