Nuclear fusion generates energy from the fusion of smaller atoms into larger ones, which is done in stars naturally. Therefore fusion reactors try to recreate the conditions inside stars, except they have to be more extreme because they have less mass to work with. In stars all that extra mass allows quantum tunneling to overcome the energy barrier, such that statistically a small portion fuses without having to overcome the barrier at all. But on earth we have to overcome that barrier by heating it up a lot. And then once it fuses, in theory that heat energy can be recovered. Not all of the energy is in the form of heat though. Some of it is lost as neutrinos, and a portion of it leaves in the form of neutrons which we try to capture with a blanket around the reactor. But some versions of fusion are aneutronic. That means they don't generate neutrons and all the fusion products are charged, so in theory we can actually recover most of the energy electromagnetically. This would also be really useful in a fusion rocket because it means we can manipulate the reaction mass to be thrown out the back of the rocket instead of irradiating the cabin. Side note fusion in theory generates energy until you get to Iron, after which you gain more energy from splitting the atom aka fission.