Pressure raises the temperature at which a given type of matter changes states, such as going from solid to liquid or liquid to gas. For example, water boils at 212 degrees Fahrenheit at sea level. At higher altitudes, the pressure from the atmosphere decreases and therefore lowers the boiling point.
This occurs because the state of matter depends on how quickly its molecules are moving, or how much energy they have. At high pressures, the molecules cannot move as quickly because they are packed more densely. The closer molecules are, the tighter of a bond they form. More energy is necessary to make them break the lattices that make a solid structure. Pressure can be exerted on gases more easily than on liquids and solids.
A pressure cooker makes use of this property of matter. By increasing the pressure inside the cooker, it increases the temperature at which water boils. Hotter liquid water and steam transfer more heat and allows food to cook more quickly than at normal pressure, and water conducts more heat than air. Conversely, at high altitudes recipes have to be adjusted for a lower boiling point and food has to be cooked for a longer time.