The spin rate is the number of times a ball rotates upon itself between the time it leaves the pitcher's hand and when it enters the strike zone. It is an important measure because the efficiency of a breaking ball is directly correlated to its spin rate, as the spin is what allows the ball's trajectory to curve. Therefore, the higher the spin rate on a curve ball, the more effective a pitch it is. In contrast, a knuckleball works by reducing the spin rate to virtually nothing, making the pitch's trajectory unpredictable.
The importance of this factor has been known for a long time, but there was little way to measure it or make comparisons between pitchers until the advent of advanced tracking systems such as Statcast. As a result of these developments, the term and the notion behind it became very popular in the second half of the 2010s.