A pressure group is basically a garden pump attached to a pressure tank. This tank is filled with water and is under pressure. When the pressure group is turned on, the pressure will drop and the water supply will be automaticcaly filled up.
The ideal part of a pressure group is that it can be used to increase water pressure of the main pipes. Because the water only has to be supplied when the pressure group is turned on and the pressure drops, the pump does not have to work continuously, saving a lot of energy.