Spin rate is measured in RPM. At the same 95 mph, a high-spin (2400+) 4-seamer 'rises' and is harder to hit.
Riding 4-seamers want 2300-2700 RPM; curveballs benefit from 2700+ RPM.
Each pitch type has an optimal band — higher isn't universally better (changeups prefer low spin).
The 2021 sticky-substance crackdown lowered MLB average RPMs.