 Pulse width modulation

Pulse-width modulation (PWM) is a technique used to encode a message into a pulsing signal. It involves varying the width of the pulses in a signal while keeping the frequency constant. This modulation method is commonly used in various applications, including controlling the power delivered to electronic devices, transmission of information, and generating analog signals.

GPIO generally only supports discrete outputs.

But for some types of hardware, like LEDs and some motors, the system has some inertia, and if you switch on and off fast enough, you get a result similar to having an intermediate voltage.

So with pulse width modulation we can fake analog output from digital output in a good enough manner.