It says that in some cases, mass of reactants will be slightly higher than the man of the product. The difference in mass is converted into energy.
eg) In the sun, 4 Hydrogen atoms combine to form a Helium atom. The mass of 4 'H' atom is 4u and the mass of 1' He'atom is not exactly 4u. It is a bit lower. So the difference in the mass is converted into energy. We can find this energy in joules by using :
where E is the energy, m is the mass and c is the speed of light.