 fusion is a nuclear reaction in which two smaller nuclei merge to build one bigger one. So we have the small one here and the small one here they merge together to build one bigger one. Sometimes there's some other small particle flying off in the process. An example could be if we have hydrogen 2 if they merge they can form helium 4. If you write down the nuclear equation for this you would have 2 times hydrogen 2 will become 1 times helium 4. As with all nuclear reactions your total mass number should be conserved so 2 times 2 is 4 as should be the total charge. So here we have a total of two protons here we have a total of two protons so that's working out. However the mass itself is not conserved. By doing this process part of the mass is converted to energy according to E equals MC, C being the speed of light, squared. Now where is this fusion happening? It's naturally happening on the Sun where hydrogen merges to form helium and the energy is distributed through sunlight all around. That's why we have sunlight. Currently there are also scientists around the world trying to work on the fusion reactor. The main advantage of a fusion reactor would be that the result of the reaction is not radioactive itself. In the fusion reactor the products of our reaction are usually radioactive. In the fusion reactor they would be completely harmless so it would be a great achievement if we get the fusion reactor to work.