The m in the equation represents the nuclear mass defect. This mass defect is the difference between the mass of the stable nucleus that was produced during the nuclear change, and the sum of the masses of its constituent protons and neutrons.
Fusion is the nuclear process which occurs in hydrogen bombs.
It is hoped that nuclear fusion will some day be used to generate cheap electrical energy for sustaining the massive needs of our world. As yet scientists have not been successful at using controlled fusion to produce electrical power for use by the public. In all of the stars throughout the universe fusion is the process which creates the tremendous amounts of energy that is released. As difficult as it may be to believe, the majority of the elements around us were created at some time in the distant past by this process. Astrophysicists do believe that the majority of hydrogen and helium in the universe was created during the Big Bang.