Digital Pulse Width Modulation, or PWM, is a digital on/off signal that pulses up and down at various frequency rates and duty cycles. The frequency rate is a measure of how often the pulse repeats and is generally expressed as "hertz" or cycles per second. The duty cycle is the percentage of time that the pulse is on (high) versus off (low). When talking about digital signals, they are either on or off which is often referred to as high or low, or simple 1 or 0. A digital PWM toggles between on and off.
Robots use PWM signals for a variety of purposes. PWM can determine the speed of a motor, the brightness of an LED or the position of an RC servo.
To understand PWM, consider how it can affect a Light Emitting Diode or LED.
Let's start with very slow PWM that has a frequency of 1Hz with a 50% duty cycle. 1Hz means that the PWM pulse repeats at a frequency of once each second and the 50% duty cycle means that the signal is on for 50% of the pulse. Of course, if the pulse on for 50% of one second, that means it is on for one half of a second and off for the other half the second. So in this case, the LED would blink every second and stay on for a half second.
Now think about how the duty cycle might affect the LED. If we speed up the frequency of the blinking, the LED will blink so fast your eyes won't be able to notice it. At 100Hz with a 50% duty cycle, the LED would pulse on and off so fast that you would barely notice anything. The LED would just look like it was on all the time.
What would happen if the duty cycle is changed to 90%? The LED would still be blinking very fast at 100 times per second but now it would on 90% of the time so it would appear brighter. Change that to a 10% duty cycle and now the LED will be on only 10% of the time. Since it is on 10% of the time, it must be off 90% of the time. Naturally, since the LED is off most of the time, it will appear very dim.