The average spin rate of the ball as it travels from the point of release until it is caught. Spin rate is measured in RPMs (revolutions per minute).
The higher spin rate generally means more break on the pitch; that is, more movement side-to-side or up-and-down.
Real Life Example:
All pitches fall due to gravity as they approach the plate, but fastballs don’t fall as much because of backspin and curveballs fall even more than expected because of top spin. So spin on the ball can mislead the batter about its path.
For position players, throws generally have backspin, and higher spin means the ball has more carry as it moves to the target.