If you want to control the brightness of a LED, the speed of a DC motor or the direction of a servo, you will need PWM.
The video shows PWM being used to control the brightness of some LEDs.
Pulse-width modulation (PWM) is used to control the amount of power supplied to electrical devices, especially to DC motors, servos and LEDs.
PWM is able to achieve this by quickly turning off and on the power to the device. The measurement for this is duty cycle.
Duty cycles describes the proportion of 'on'; a low duty cycle corresponds to low power, because the power is off for most of the time. A high duty cycle corresponds to high power, because the power is on most of the time.
Duty cycle is expressed in percent, 50% is when the power is on half the time and 100% being fully on.