Compressibility can be simply defined as the ability of a certain substance to be shrunk to a smaller volume when force is applied. It is a change in the relative volume of a particular fluid or solid as a response to the applied pressure. We can relate this with the Boyle's law equation: P1V1 = P2V2. As you decrease the volume in a certain closed area, you increase the amount of force that you need to apply.
For example in syringe, we can change the volume of it by simply pulling and pushing the plunger top (end fitting). As you pull the plunger, you are getting an amount of volume of any substance. To exhibit the compressibility, cover the needle and ensure that no substance is going to leak. Push the plunger. If you are able to push it, it means that the substance is compressible.
Generally solid substances are the hardest to be compressed, followed by liquids and the gasses.