Most traditional generators (think CCGT, nuclear, hydro) run in synchronous mode. Which means the generators spins with rpm proportional/equal to the network frequency. For 50Hz that would be 3000rpm.
When the total electricity demand exceeds the supply, the kinetic energy of spinning generators is converted to electricity which slows down the generator rpm, which reduces the frequency. All synchronous generators on a given network spin at the same frequency, so when the demand is too high all of them slow down a bit.