The electric grid needs to run at certain voltage ( 110v /230v at the home ) and a certain ac frequency (50hz eu or 60hz us).

When a (steam) generator runs it produces a certain voltage at a certain frequency depending on the load and rpm.

More load than output and the voltage and frequency(generator goes slower) drop, more output than load voltage and frequency go up.

Electric grid operators turn on/off generators or demand more/less load depending on voltage and frequency the generator are putting out.

Allowed voltage range is +-6% (+10%-6% in eu) they try to keep the Frequency in a +-0.2% ish range.