The nuclear binding energy is actually nothing but the energy that is used to breakdown a nucleus into its constituent units, the protons and the neutrons.
The whole nucleus weighs a lot lesser than the combined mass of the individual protons and neutrons.
When the neutrons and the protons forms a nucleus(nuclear fusion) or when a nucleus is split to give out the individual components(nuclear fission), a specific amount of energy is expelled
This energy is represented by the mass that is missing, the difference between the mass of the nucleus as a whole unit and the combined mass of the constituent units; This is called the mass defect.
