Compressibility is the property of being reduced to a smaller space by pressure. This property is a consequence of porosity, and the change of bulk comes from the particles being brought closer together by the pressure.
The compressibility of fluid is basically a measure of the change in density that will be produced in the fluid by a specified change in pressure. Gases are, in general, highly compressible whereas most liquids have very low compressibility.
The fact that compressibility effects can have a large influence on fluid flow. The study of these flows in which the changes in density and temperature are important is basically what is known as compressible fluid flow or gas dynamics.