First the compressibility factor Z= PV/nRT
For the ideal gas, the compressibility factor is Z = 1.
But the real gases The value of Z generally increase with increasing pressure:
1- At high pressure (the gas hard to compress): attractive force is weaker than the repulsive force for the gas, and its real molar volume gets larger than the ideal molar volume, and that means z >1.
2- At low pressure (the gas easy to compress): attractive force is stronger than the repulsive force for the gas, and its real molar volume gets smaller than the ideal volume, and that means Z<1.
