Every object is made up of tiny particles called atoms. Most of its mass is concentrated in centre(nucleus, comprising proton and neutron), and rest of the mass is in the cloud of electons surrounding nucleus. There is enormous energy in the bonds which holds the atom skeleton together. This energy is utilized to generate nuclear energy. There are two process to exploit this energy:
1) Fission- energetic splitting of large atoms like Uranium into two smaller atoms with release of shockingly large amount of energy. It is done with neutrons which hit atoms and split them. One splitting atom can cause nearby ones to split, producing a nuclear chain reaction of sustained energy release. To occur reaction in controlled fashion, moderators like heavy water are used which absorb extra neutrons(malfunctioning of which could cause nuclear explosion and disaster).
2) Fusion- combining of two small atoms like hydrogen or helium to produce heavier atoms and energy. This reaction can produce much larger amount of energy. The fusion reactions are behind the enormous sustained energy of sun. However this technology has not been commercially developed yet.
In both the processes from reactant to product there is net loss of mass of atom. This loss in mass produces energy following the well known Einstein formula E=mc^2 where m is change in mass and c is speed of light.
I hope this make sense to you ?