[avidiax]

There is conservation of energy. Energy in strictly equals energy out.

The electrical grid is a bunch of heavy spinning motor-generators that are electrically connected to heavy spinning motor-generators and other loads like lightbulbs. The motor-generators are electrically identical, except that we expect to add energy to one side and extract energy on the other*.

So what happens if the energy added by power plants is less than the energy extracted by lightbulbs and the loads on the motor-generators? Conservation of energy means that we must get the energy by slowing down the generators, extracting their kinetic energy. That lowers the grid frequency.

The same thing can happen in reverse to increase the grid frequency. Too much power generation must increase the kinetic energy of the motor-generators.

* Many of the loads on the grid are intentional or unintentional flywheels, so they may actually add energy to the grid if the grid is slowing, increasing stability.