Yep, sounds like a expansion/pressure vessel. Does exactly what I've just said but your one will deliver quite a large amount of water, which is handy if you have a power cut. It may also have a pressure switch on it, so that when your store has reached say 4 bar it will shut off then when it depletes to maybe 2 bar it will kick the pump in and replenish the tank, this stops excessive cycling of the pump (therefore less wear) and keeps a relatively constant supply to the house etc.. Run a tap and see what the pressure gauge on top does, is the gauge on a small box with a wire or 2 coming out o