LIBRARYPWM: a fake analog output.

A microcontroller pin is digital: it can only be fully on or fully off. But switch it on and off fast, with a controllable on-fraction, and it averages to a level in between. That is PWM, for pulse-width modulation, and it is how one digital pin dims an LED or sets a motor's speed without any analog output hardware.

Duty cycle: the on-fraction

The duty cycle is the fraction of each cycle the pin spends high. At 25 percent duty the output averages a quarter of the supply; at 75 percent, three-quarters. Sweep the duty cycle and the average voltage follows it smoothly, which is the whole trick.

Frequency: fast enough to blur

The switching frequency has to be fast enough that whatever you are driving cannot follow the individual pulses. Your eye blurs a fast-blinking LED into a steady brightness; a motor's inertia smooths the pulses into a steady pull. Too slow, and the LED visibly flickers or the motor whines at the switching rate. Fast enough, and only the average shows.

What PWM drives

PWM sets LED brightness, motor speed, a servo's position (through a specific pulse width), a buzzer's tone, and, once you smooth it with an RC filter, a rough analog voltage. On the ESP32 the LEDC peripheral generates PWM in hardware on any output pin, so your code just sets a duty cycle and the hardware toggles the pin at the frequency you chose, with no CPU effort per pulse.

The fundamentals guide on reactive parts and filtering shows the RC low-pass that averages a PWM signal into a real, steady voltage, if you want the analog output rather than just the blinking.