The frequency drifts up and down whenever demand doesn't exactly match supply. Higher demand slows the frequency down, higher supply speeds it up. This is actually the main way power companies know if supply and demand match, and if power stations have to ramp up or down.
The frequency changes are pretty small in normal operation, but on a clock that uses the frequency to keep time they accumulate. They only work reliably because power companies know about them and occasionally deliberately run a bit over or under capacity to make the average match again.
Fun fact, there are databases of the exact frequency vs. time and it can be used to accurately time stamp audio/video recordings by correlating the ~50/60hz noise in the recording with the database. Good writeup on the technique and how it has been used in court cases: https://robertheaton.com/enf/
With a lot of simplification, consuming electricity acts as a brake on giant wheels inside the power plants that are usually spinning at mains frequency. The plants accelerate the same wheels, so with much demand the braking wins and with too little the acceleration wins.
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.
Because generators -- where virtually _all_ AC power is created -- start running slower with high demand. They catch up, via increased power input through governors, but changes in load will necessarily have some impact on speed.
The frequency is generated by rotating electrical generators. Higher electrical demand increases the mechanical load on the generator, making it rotate more slowly, producing a lower frequency.
The frequency changes are pretty small in normal operation, but on a clock that uses the frequency to keep time they accumulate. They only work reliably because power companies know about them and occasionally deliberately run a bit over or under capacity to make the average match again.