Whenever two or more objects are bound together by strong forces, there is a binding energy – the energy needed to pry these objects apart. Since Einstein, we know that energy and mass are equivalent. To this binding energy there corresponds a mass. It is called the mass defect because, by this amount, the mass of the component object is less than the sum of the masses of its parts.
Some more information about binding energies and the mass defect can be found in the spotlight topic Is the whole the sum of its parts?