Some compounds contain high energy bonds. The high-energy bond is the one between the last two phosphate groups. This bond has high energy because both of the phosphate groups contain several oxygen atoms, and the electronegative oxygen atoms tend to pull electrons toward themselves. The resulting tension produces a high energy condition. When this bond is broken as the phosphate group is removed, energy is released.
Free energy is the energy that can be made available to do work.
Essentially, this is the amount of energy that can be released by breaking chemical bonds in a molecule. These reactions that release energy are exergonic. When the products of a reaction have more free energy than the reactants, energy must be added. These reactions are endergonic. Reduction reactions and phosphorylation reactions are endergonic.