Variation of binding energy with mass number.
Binding energy is the amount of energy released when a particular isotope is formed. Protons and neutron are held together by the strong force, which only acts over very small distances but is able to overcome the electrostatic repulsion between protons. The strength of the bonding is measured by the binding energy per nucleon where "nucleon" is a collective name for neutrons and protons (sometimes called the mass defect per nucleon). The mass defect reflects the fact that the total mass of the nucleus is less than the sum of the mass of the individual neutrons and protons that formed it. The difference in mass is equivalent to the energy released in forming the nucleus.
The general decrease in binding energy beyond iron is due to the fact that, as nuclei gets bigger, the ability of the strong force to counteract the electrostatic repulsion between protons becomes weaker. The most tightly bound isotopes are 62Ni, 58Fe, and 56Fe, which have binding energies of 8.8 MeV per nucleon. Elements heavier than these isotopes can yield energy by nuclear fission; lighter isotopes can yield energy by fusion.