The degree of binding of the ligand with the protein refers to the binding affinity. The energy released due to the bond formation, or rather, interaction of the ligand and protein is termed in form of binding energy. The free energy of the favourable reaction is negative. Lesser the binding energy, better is the binding of the ligand and protein.
Binding free energy is the energy released due to the bond formation or interaction of protein-ligand. The Binding free energy at the active site can be used as an estimate of binding affinity. The lower the free energy the tighter the binding and the affinity.