$\left( P +\frac{ n ^{2} a }{ V ^{2}}\right)( V - nb )= nRT$
At low pressure, when the sample occupies a large volume, the molecules are so far apart for most of the time that the intermolecular forces play no significant role, and the gas behaves virtually perfectly.
$a$ and $b$ are characteristic of a gas and are independent of temperature.
The term $\left( P +\frac{ n ^{2} a }{ V ^{2}}\right)$ represents the pressure exerted by an ideal gas while $P$ represents the pressure exerted by a real gas.