Which property of molecule of real gases is indicated by van der Waal's constant 'a'?

The unit of van der waal’s constant ‘a’ is :

The Compressibility factor for one mole of a van der Waal's gas at '0^circ C' and '100 atm' pressure is found to be '0.5 .' Assuming that the volume of gas molecule is negligible, calculate the van der Waal's constant 'a'?

What are van der Waals' forces?

Van dar Waal's equation explains the behaviour of rea gases. What does the van der Waal's constant 'a' indicate?