It is an equation that relates equilibrium constant to temperature.
Equilibrium constants can change according to temperature. The Van't Hoff equation defines the change in equilibrium constant as a function of temperature. The equation under standard conditions is defined as:
One of the main applications of the Van't Hoff equation is to make estimations of the equilibrium constant at a specified reaction temperature. To do this, enthalpy of reaction is assumed to be constant over the temperature range considered. It can be a useful exercise, for example in industrial applications where chemical engineers need to check what improvement in yield can be achieved by raising the reaction temperature of an equilibrium system.