Essentially a throttle, but more likely a demand based system, such that non-busy processors can run at the lowest possible speed and voltage, and when work stacks up, it ramps up.
Great for the smart phone in your pocket which has nothing to do for hours at a time other than check the email and listen for calls.
Since its screen is off, you really don't care how fast it does those things as long as they are just barely fast enough.
There is a great deal of "stare time" that happens when people look at computers, and the processors are spinning away all the time while you are reading this. They could just as well drop to an extremely low power state, and wait for a mouse move, finger tap, or something else.
This much we've been doing all along, for the last 20 years. But power consumption still remained high, because even simple tasks like checking the clock to see if its time to increment that digital time read out took processing power, and historically any use of the processor kept it awake at something like full power for that task.
Now, those tasks can be performed at extremely low power, without ramping up the speed. Only when the processor can't meet the demand would the system increase the voltage and speed up the chip.