Refrigeration means to extract heat. A refrigeration system extracts heat at low temperatures and rejects it to the higher ambient temperature.
This is done by the help of a working fluid called the refrigerant. This fluid evaporates at a low pressure and temperature and extracts the heat from the medium that have to be cooled. Later on, the vaporized refrigerant has to be liquified again, this is done by increasing the pressure high enough to be able to condensate at ambient temperature.
The increase of pressure is done by a compressor which needs mechanical energy to be driven. This driving energy is normally electricity but it could also be mechanical energy from an engine or a turbine.
