The difference in vertical location at the plate from a pitch that was thrown with no spin compared to the actual pitch that was thrown.
From the pitcher’s view a pitch with no spin will move in a straight line horizontally to the plate. A ball will also travel with no vertical movement if the spin on the ball is perfectly 3 o’clock or 9 o’clock.
If the spin direction is between 3:01 and 8:59, the ball will have a vertical break downward.
If the spin direction is between 9:01 and 2:59, the ball will have a vertical break to the upward.
Real life Example:
A fastball will have a large vertical break upward. A curveball will have a large downward vertical break.