This is a phrase used to describe a skilled hitter in baseball capable of hitting the ball where they want to. In general, this refers to being able to hit to the right, left, or center of the field. If a player is capable of hitting the same pitch to either side of the field, they would be considered a place hitter. Being able to place ones hits is can be an extremely difficult skills to learn and execute. There are a lot of batters that can only hit different types of pitches to one side or the other.
In order to be a place hitter, a player must have a mastery of many different skills. The must be able to tell what type of pitch they are receiving based on information they pick up from the pitcher. They must also be able to process that information and adjust their swing accordingly. Beyond that, a player must understand what adjustments to make in order to hit the ball to a given area of the field. For example, if a right handed player wants to hit the ball to right field on fastball, they would need to swing earlier than if they were being pitched a curveball.