The matter is everything that has mass and occupies a place in space. This is composed of atoms, tiny particles that, in turn, are composed of even smaller ones, called subatomic particles, which are grouped to form differents objects. 

Under non-extreme conditions of temperature, matter can be represented in three diferents physical states: solid state, liquid state and gaseous state. The different among each state depends on the proximity among the atoms that compose it, being the solid the state whose atoms are closer together and the gaseous state with more free atoms.

Already knowing all this, we can define the compression as the increase of the pressure on the atoms, in this case of a gas, thus reducing its volume and increasing the temperature of that gas.