Below the critical temperature, for Van der Waals isotherms exhibit unphysical oscillatory behavior. Maxwell's construction corrects this by replacing the oscillating segment of the isotherm by a horizontal line, which is interpreted as belonging to a two-phase system, with liquid and vapor in equilibrium. The Maxwell construction is derived by arranging the two shaded regions produced by intersection with the Van der Waals isotherm to have equal areas. You can show the Maxwell construction by checking the box.