It's the standard way of varying the power of a heating element - you turn it ON for a while, then OFF for a while - the ratio of the two is the power you're applying to the load (it's basically slow PWM).
So if you had a 1000W heating element, and you turned it ON for 5 secs, then OFF for 5 seconds, the load would receive 500W.
The thermal inertia of the element or load smooths the pulses out.
An obvious way to do it with AC mains to count the number of mains cycles, and turn ON and OFF accordingly.