Frequency is the number of cycles per second. Its unit of measurement is hertz [Hz].
Power distribution grids use alternating current (AC). Alternating current is easily transformed to various voltage [V] levels.
Continental Europe uses a synchronous 400/230V 50Hz power grid. In North America, power is supplied to households at 120V and 60Hz.
The grid frequency is generally very stable in Europe and stays within 49 to 51 Hz.
In the context of NETIO products
NETIO smart sockets measure the power grid frequency as a single value common for all sockets.