Fusion happens when two lightweight atoms. are forced together to form a heavier one, and a lot of energy is produced as a result. However, fusion can only occur at the incredibly high temperatures and pressures found at the centre of stars.
All chemical elements in the universe heavier than Hydrogen and Helium were created in stars through nuclear fusion. At the end of the star's lifetime, these heavy elements can be re-distributed into space and nearby gas clouds or nebula, and so form the building blocks for future generations of stars.
This means that you, and everything around you, must be made of "stardust".
Scientists are trying to find a way to make nuclear fusion power stations, since they would be much cleaner to run than our current nuclear fission power stations and they would produce a lot of energy. However, this is very difficult to do on Earth because you have to make the "fuel" very hot and squeeze it down into a tiny space to get fusion to work.