PWM

Pulse-width modulation is a commonly used technique for controlling inertial electrical devices, made practical by modern electronic power switches.

One way a PWM can be used is to control the speed of a electric motor.  You could use a resistor but it would get hot and excess heat means wasted energy.  You could give the motor full power but in short fast pulses which will make the motor behave as if it was getting a steady voltage.