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 thrown within +/- 2 mph of velocity and +/- 0.5 feet of extension and release point.
For example, look at the 2021 curveball drop leaders, with a minimum of 250 thrown. Dylan Cease got 61.7 inches of drop overall, less than Adam Wainwright, whose curve had 66.5 inches of drop. However, Cease generated +8.6 inches of drop more than average, while Wainwright generated +3.0 inches of drop vs. average. In other words, Cease created more drop vs. comparable curveballs.
That difference between "total drop" and "drop vs. average" occurred because Wainwright threw his curveball over 6 mph slower than Cease. Cease's curveball averaged 79.9 mph, while Wainwright's averaged 73.4 mph, giving it more time to drop. On the other hand, Cease's high-spin curve (he averaged 2,754 rpm in 2021) creates sharp vertical break. Measured against the comparable curveballs to theirs, Cease had better drop vs. average than Wainwright.
Statcast's Pitch Movement leaderboard on Baseball Savant displays pitch movement data for each distinct pitch type going back to the 2017 season.
For breaking balls, offspeed pitches and sinkers, more vertical movement vs. average means more the pitch is generating more "drop." However, for four-seam fastballs, better vertical movement vs. average means the pitch is generating more "rise."
Fastballs do not physically rise on their way to the plate, but a "rising fastball" is one that drops less than the hitter would expect, generally due to being thrown with high spin rate and true backspin. A four-seam fastball with rise is more likely to generate swings-and-misses. Therefore, four-seam fastballs on the Pitch Movement leaderboard are ranked according to their rise vs. average.
How to read the Pitch Movement leaderboard
"In 2022, Seth Lugo's curveball dropped 60.8 inches. That was 8 inches more drop -- or 15% more drop -- than similar MLB curveballs based on velocity and release point. Lugo had the most added drop of any pitcher who threw 250 curves."
"In 2019, Gerrit Cole and Justin Verlander each generated +2.9 inches of rise above average on their four-seam fastballs -- the most rise among pitchers who threw at least 1,000 four-seamers. Cole's four-seamer averaged 97.1 mph and only dropped 10.9 inches on its way to the plate, while Verlander's four-seamer averaged 94.6 mph and only dropped 10.7 inches on its way to the plate."
"In 2022, Lou Trivino's slider averaged the most total horizontal movement of any qualifying slider in the Majors, breaking an average 16.8 inches. However, Matt Brash generated the most horizontal break above average, as his slider got +10.2 inches of horizontal break vs. comparable big league sliders."