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. Now, to calculate mathematically the gives free energy of this reaction, all we have to do is take the free energy of the products and subtract the free energy of the reactants. And that gives us the gives free energy given by Delta G. So this quantity here is how much energy is going to be released in this reaction. And it's basically how much energy we can use in some process.