The power supplied through our wall sockets is actually a flow of jittered electrons. These electrons have been provided by the power plants with such a power surge that they vibrate 50 times per second. In the world of electricity, this is what is referred to as 50 Hertz. The entire European power grid, as well as all equipment, devices, machines and systems related thereto are based on this principle.
It is important to maintain this 50 Hertz standard for the entire synchronously operated grid. Within ENTSO-E there are 5 of these synchronous areas. If the frequency differs significantly within one of these areas - either upwards or downwards - this will result in power failures and in the end even in blackouts.
Frequency can be compared to a steel spring: if you pull at one end of the spring, the spring will extend; and will "dive" below 50 Hertz. However, if one side is pushed in, the spring is compressed and the frequency increases. If we project this image on power supply, we must realise that in the European grid hundreds of producers are "pushing in" at the exact same time, while hundreds of millions of European consumers are "pulling" at the other end at the exact same time. It should continuously be ensured that pulling as well as pushing in is at the same level at all times, for only then the 50 Hertz standard can be maintained. It is the responsibility of the TSOs to monitor the balance between production and consumption, and this includes the balance on international level.
The complication here is that power cannot be stored in large quantities; the quantity which is extracted from the grid should be produced at the same time, meaning it is a delicate balance.