Nuclear fusion releases energy due to a phenomenon called the mass-energy equivalence, as described by Einstein's famous equation, E = mc². In nuclear fusion, the nuclei of atoms combine to form a new nucleus, and during this process, a small fraction of the mass is converted into energy.
When two light atomic nuclei combine to form a heavier nucleus, such as in the case of hydrogen nuclei (protons) fusing to form helium, the total mass of the resulting nucleus is slightly less than the combined mass of the original nuclei. This difference in mass is known as the mass defect.
According to Einstein's equation, this mass defect corresponds to a certain amount of energy. The released energy is a result of the conversion of mass into energy. The energy is released in the form of high-energy photons, such as gamma rays, as well as the kinetic energy of the newly formed nucleus and any emitted particles.
The energy released during nuclear fusion is enormous because it is based on the conversion of a small amount of mass. This energy release is what powers stars, including our Sun, as they continuously undergo nuclear fusion reactions in their cores.
Regarding your second question, in a nuclear fusion reaction, the total mass of the resulting nucleus is slightly less than the sum of the masses of the original nuclei. This mass deficit is converted into energy according to Einstein's equation, E = mc². The energy released accounts for the missing mass and is not dependent on the individual masses being perfectly equal. The overall energy released is determined by the difference in total mass between the initial nuclei and the resulting nucleus.