This is a baseball phrase used to describe a ball hit initially in the air in fair territory, only to curve, or "hook" towards foul territory. This is also sometimes known as a "slice foul." Typically, a ball of this nature will be hit down the first or third base line, and then hook into foul territory. A hook foul ball may also be the result of wind gusts or peculiar spin placed on the ball by the hit. If the ball were hit closer to the center of the field, the same hit would most likely stay in fair territory.
A player who hits lots of hook fouls may have an issue with their swing. If a player swings too early, too late, too high, or too low, but still makes contact with the ball, the ball will come off the bat with a spin that helps push the ball foul. In order to avoid hitting hook foul balls, hitters hit the ball as close to the sweet spot on the bat as possible. This is also known as hitting the ball, "square."