PWM (Pulse Width Modulation)

Read more about Pulse-width_modulation on wikipedia.

PWM stands for Pulse Width Modulation. A PWM signal is a square wave that transmits data by varying the width of the signal, or time which it is high, creating an analog signal with limits, so to speak (any value between 0 and 1 inclusively). By varying the width of the signal, the speed controller can make changes to the speed the motors are run at.