Nuclear fusion is the joining of small nuclei to make a larger nucleus. For example, two hydrogen nuclei (isotopes of hydrogen) can fuse to make a nucleus of helium. Energy is released in the process.
If you add up the masses of the nuclei that go in, the total is slightly more than the mass of the single nucleus that comes out. A tiny amount of mass has been lost. This lost mass is not destroyed. It is transferred as energy, which is released as kinetic energy of the products and as radiation. Even a very small loss of mass releases a huge amount of energy, which is why fusion is so powerful.
Fusion is the energy source for stars. In the core of a star such as the Sun, hydrogen nuclei fuse to make helium, and the energy released is what makes the star shine. Stars stay stable for billions of years because the energy released balances the inward pull of gravity.