To be able to properly make use of Controlled Nuclear Fusion, humanity first had to solve numerous sub-technologies independently. One major challenge was developing Extreme Heat-Resistant Materials, as the reactor walls needed to withstand temperatures exceeding 150 million°C without breaking down.
There was also Ultra-High-Purity Fuel Processing, which denoted extremely efficient extraction and refinement of deuterium/tritium. These two were the precious fuels used for Nuclear Fusion, and their utility was quite simple.
Deuterium (²H) was a hydrogen isotope with one proton and one neutron.
Tritium (³H) was another hydrogen isotope, but with one proton and two neutrons.
So, when deuterium and tritium fuse together in Nuclear Fusion, they form helium (⁴He) and release a single neutron, along with a huge amount of energy which can then be converted into electricity.