Compressibility is the change in volume due to the change in pressure of a fluid.
It is the ability of molecules in certain fluids to get compacted under pressure. When the pressure is reduced, the molecules disperse and spread out again, increasing the volume.
Gases can be compressed by increasing the pressure.
Liquids cannot be compressed. The change in volume due to the change in pressure is very small in liquids.
Gases are compressed by special devices called compressors.