From the name itself, you can guess that a nuclear fusion refers to the joining of two small nuclei in order to form one big nucleus. Nuclear fusion can happen to starts and it is quite important. A star goes through a lot of temperatures and pressures. There are times when the fusion would be enough to provide light for millions of years.
This also explains why there are still some stars that are visible in the night sky even though they have died for a long time. While a star is alive, hydrogen nuclei would fuse together so that helium nuclei can be achieved. The energy output that stars produce are based on nuclear fusion.