The Van der Waals equation of state extends the ideal gas law to include the effects of interaction between molecules of a gas, as well as accounting for the finite size of the molecules. This is an example of a cubic equation of state as it can be rearranged as a cubic polynomial in terms of the molar volume of the gas.
The ideal gas law treats gas molecules as point particles that interact with their containers but not each other, meaning they neither take up space nor change kinetic energy during collisions. To account for the volume occupied by real gas molecules, the Van der Waals equation includes a term $b$ for the volume occupied by the actual molecules. It also has a second modification to account for the interaction between the molecules of the gas, $a$.
The Van der Waals equation is typically given as, $$P=\frac{RT}{V-b}-\frac{a}{V^2}$$
The constants $a$ and $b$ can be expressed for different fluids in terms of the critical constants as,
$$a=\frac{27}{64}\frac{R^2T_c^2}{P_c}$$
$$b=\frac{1}{8}\frac{RT_c}{P_c}$$
This equation approximates the behaviour of real fluids above their critical temperatures and is qualitatively reasonable for their liquid (left hand side of the graph) and low-pressure gaseous states (right hand side of the graph) at low temperatures. However, near the phase transition where the liquid phase and the gas phase are in equilibrium, the Van der Waals equation fails to accurately model observed experimental behaviour, i.e. should be a region of constant $P$ with changing $V$.
The true behaviour in this region can be approximated from the Maxwell equal area rule. This is where a horizontal line (isobar) is drawn in the oscillatory region, where the area above the line and below the van der Waals curve is equal to the area below the line and above the van der Waals curve.
In the graph below, the dashed line is the Van der Waals equation in the equilibrium region with the horizontal line being the equilibrium behaviour given by the Maxwell equal area rule. If there is no horizontal line, then the fluid is above the critical temperature and no vapour-liquid transition occurs.