The movement of a pitch is defined in inches, both in raw numbers and (more importantly) as a measurement against average. It is displayed separately for horizontal break and vertical drop.
As opposed to other available pitch movement numbers that remove gravity, Statcast's pitch movement numbers are displayed with gravity, in an attempt to better align with the real-world eye test, as pitches are of course affected by gravity.
Since gravity requires time, and slower pitches aren't "better" just because they have more time to move, the movement of a pitch is compared to "average" movement, by comparing it to other MLB pitch types within +/- 2 MPH and from within +/- 0.5 feet of extension and release.
For example, look at the 2019 curveball drop leaders, minimum 500 thrown. Trevor Bauer got 63.6 inches of drop, while Max Fried got 69.6 inches of drop. However, Bauer got 9.5 inches more drop than average, while Fried got 7.7 inches more drop than average -- because Fried's 74.5 mph curve was nearly 5 mph slower than Bauer's 79.1, giving it more time to drop. Against their comparable averages, Bauer had better drop vs. average.
In A Call"In 2018, Trevor Bauer's curveball dropped 64 inches. That was 9.3 inches more drop -- or 17% more drop -- than similar MLB curveballs at his velocity, which was the most added drop of any pitcher who threw 200 curves."