Nuclear physics is the branch of science which looks inside atoms, to their nucleus, and sees what effect different interaction have on it.
These different interactions fall under two main categories; nuclear fission, where the nucleus breaks apart, and nuclear fusion, where the nucleus gets bigger by fusing, or 'gluing', together with another nucleus. Both of these processes release energy.
A nucleus sits inside an atom and contains two different particles known as neutrons and protons. It wasn't discovered until 1911 by Ernest Rutherford, a physicist at Oxford University.
The proton has a positive charge, and the neutron has no charge, and between them they make up most of the mass of an atom.
These small particles have an enormous impact on the Universe we see, as all stars we see are powered by nuclear fusion during their lifetimes. It is the fusion of nuclei which gives the star energy.