Unlike solids and liquids, gases have the ability to expand and fill a container. As a result of this expansion, there are great amounts of empty space between the particles of a gas. The reverse of expansion, compression, is also a property that is specific to gases. Compressibility is a measure of how much the volume of a gas can be reduced under pressure.
Recall, that air pressure is the result of the billions of collisions between the gas particles and the walls of the container. Pressure can be affected by four variables: volume of the gas, temperature of the gas, and number of moles (amount) of the gas.