Binding energy is energy that is lost when a nucleus is formed from protons and neutrons . A lot of binding energy means a stable nucleus. A similar idea exists in ordinary chemistry. When sodium reacts with chlorine, a lot of energy is released. The result is a very stable compound - sodium chloride.
Energy has mass.
Losing energy means losing mass. In ordinary chemistry this change is too small to measure, but the amounts of energy involved in nuclear processes are so great that the change in mass is quite easily measurable. To calculate binding energy we add together the masses of all the protons and neutrons that go to make up the nucleus. The total is greater than the actual mass of the nucleus. The decrease in mass when the nucleus is formed is the mass of the energy released.