Pressure cooking is a method of cooking in a sealed vessel that does not permit air or liquids to escape. Since water's boiling point is affected by the atmospheric pressure, the pressure built up inside the cooker allows the liquid in the pot to raise to a temperature higher than 100°C before boiling. The higher temperature causes the food to cook faster. It is often used to simulate the effects of long braising or simmering in shorter periods of time.
Pressure cooker is often used by mountain climbers to compensate for the low atmospheric pressure at high altitude. Without it, the water would boil off before reaching 100°C and leaving the food improperly cooked.