As you may know, ideal gas law is equated as 

PV = nRT, where P = pressure of the gas, V = volume occupied by the gas, n = no. of moles of the gas, R = gas constant and T = temperature.

However, this expression takes into assumption that the molecules of gases interacting are point particles which undergoes elastic collisions. But, inrelaity, the existing gas molecules are not so ideal in nature and definitely does not undergo elastic collisions. 

So, to take into account the molecular sizes of such molecules and also the molecular interaction forces, van der Waal modified the above equation introducing two new constants in it. Van der Waals equation can be written as - 

where a and b are Van der Waals constants; where a is a measure of the average attraction between particles, and b is the volume excluded by a mole of particles.