Nuclear fusion is a way of producing energy by joining two light nuclei together to get one heavier nucleus. It is nuclear fusion that powers the stars!
One simple fusion reaction is to join two nuclei of heavy hydrogen (deuterium) together to make one nucleus of helium.
The reason that energy is produced is that the mass of the two deuterium nuclei is slightly greater than the mass of the one helium nucleus.
This tiny difference becomes energy.
Deuterium is found in water (since a water molecule is made of two hydrogen atoms and one oxygen atom) and in fact one atom in every 5000 atoms of hydrogen in sea water is deuterium.
This means that there is a vast supply of deuterium as heavy water that could be used for fusion.