Imagine a system made of 4 pieces, each of mass m. Einstein pointed out that the total mass of the system should be Mc² = ℰ + 4mc²

Here ℰ is the total internal energy of the system, the sum of the kinetic and potential energies of the 4 particles. But for systems in nature, the size of an atomic nucleus or larger, the total internal energy of a bound system is negative. (The trick is that if the potential energy between constituents approaches a constant as r approaches infinity, we set that constant to zero.) Notice that this means that M is less than 4m! Physicists define the binding energy of a system as B = -ℰ, so that B is the work it would take to disassemble the system completely.    Example: suppose each particle has a mass of 10 MeV/c², and the mass of the system is measured to be 30 MeV/c².  What is the binding energy of the system? ℰ = Mc² - 4mc² = 30 MeV - 40 MeV = -10 MeV.  It would take 10 MeV of work to pull the system apart.