Pulse-width modulation, is varying the duty cycle of a rectangle wave, usually with a constant frequency.
PWM is often used to control continuously varying power. Because the switching device (often a MOSFET) switches between fully off and fully on it dissipates little power (in the off-state the current is near-zero, in the on-state voltage is near-zero). If the switching frequency is high enough the load will only see the average level, which can be varied between minimum and maximum by changing the duty cycle from 0 % to 100 %.
PWM can also be used to create an analog voltage from a digital device, like a microcontroller. In that case the PWM output must be filtered by a low-pass filter to remove the switching frequency, and only retain the DC component.
Further reading
Filtering PWM Signals
