In a nuclear bomb, the famous equation E=mc[SUP]2[/SUP] comes into play.
The nucleus contains a lot of energy, but in the form of mass. Uranium has different isotopes as well. It was then known that neutrons can be used to make the heavier isotope of Uranium into lighter one. And after a nuclear explosion there is a little mass difference. That mass difference is because the mass has been converted into energy according to the energy-mass equation by Einstein. This is a very very rough explanation. You could use internet for a better and comprehensive explanation.
A fission reaction of a Uranium nucleus yields approximately 200 MeV. This is because in the energy mass equation, the light speed is so great and then the square on it, that it gets high.