It's a Variable Frequency Drive (VFD). It can take the input frequency and modify to a higher or lower level of any input source (generator or grid). We use them a lot on center pivot irrigation with variable speed motors. Instead of adjusting the voltage of a fixed votage AC motor that is designed only for 480 volts, we lower and raise the frequency of the power to slow down or speed up the motor. VFD's are also commonly seen used with Boiler pump motors to change the flow rate again not by changing the voltage but but changing the frequency of the power going to the motor.
That's variable speed control. My variable speed well pump is digitally controlled by varying the frequency. It was this system that clued me in that there was a problem when I flipped the trans switches to start the well controller. It would throw up a a big FAULT alarm but didn't really explain what the issue was. Obviously the generator was suspect.
I didn't mean to start off on a tangent, but this experience got me thinking how I mostly take frequency for granted.