This refers to a pitch count in baseball that favors the batter. The pitch count is the number of balls and strikes against a batter in a single at-bat. At least three pitches should be thrown before determining how to describe the count. If there are at least two more balls than strikes, the count may be considered favorable to the hitter. This count is favorable to the hitter because it allows them to be more selective in choosing a pitch to hit, and puts pressure on the pitcher to throw strikes to get the strikeout.
The phrase "hitter's count," has a polar opposite in "pitcher's count," which describes a situation in which the pitcher has the upper-hand; usually two strikes. When a hitter has a favorable count, the pitcher is said to be, "in a hole," because they must now throw more pitches in order to get the batter out. Players capable of consistently getting into a hitter's count situation are valuable to a team because it wears the pitcher down and forces them to be very careful with their pitches around these batters.