If you suffer a burst water pipe, you would use the stop cock to shut off the water until you can have the burst pipe repaired. The stop cock is simply a valve that you can manually turn on or off. The main stop cock is usually near were the water pipes enter the house from outside - mine is in my kitchen. There may be a stop cock on the header tank in the loft, or on the inlet pipe to the hot water tank, or in some other places, such as below the bathroom sink, and so on. When things go wrong with your water supply inside your home, you can't have too many water stop cocks!