Well, the gives free energy, loosely speaking, describes how much energy can be used in that chemical reaction. So let's suppose we have the following graph. So the y axis is the energy value and the x axis is the reaction progress. So these are the reactants here and the energy value of the reactant is somewhere here. Now, the products have a free energy value that is equal to somewhere here. And notice that the products have a lower free energy than the reactants.