Question

What property of molecules of real gases is indicated by vander waals constant?

Answer 1

there are two van der Waals constants- a and b.

a and b rise due to "interaction between gas molecules".

and due to the fact that "gas molecules do occupy volume in a container."

Even a molecule has a countable volume.

Answer 2

You didn't mention the constant...so I'm just saying here about constant "a" :-
-'a' in the vander waals equation is a measure of the magnitude of the attractive forces present between the molecules of gases. 
-The greater is the intermolecular forces, greater will the value of a. 
-Hence 'a' defines the extent of forces of attraction and repulsion between the molecules of gases.