In nuclear fusion two light nuclei join together to form one heavier nucleus. In the Sun and other stars, hydrogen nuclei fuse to make helium nuclei, and this is the source of the energy that stars give out.
Nuclei are all positively charged, so they repel each other strongly. To fuse, they must move at very high speed so that they get close enough to overcome the repulsion. This needs a very high temperature, of millions of degrees, and a very high pressure. Stars have both. On Earth, it is very hard to create and hold these conditions, so fusion is not yet used to generate electricity.
The nucleus formed has slightly less mass than the total mass of the nuclei that joined. The missing mass has been converted into energy, which is carried away as the energy of radiation. Even a very small loss of mass gives a very large amount of energy.
