Fission is where large elements, like uranium, are converted into smaller atoms – the total mass off the parts is smaller than it was at the start (so its not like a chemical reaction), and the “missing mass” has been converted into energy. The amount of energy produced can be calculated using the famous E=mc^2 formula.
Nuclear fusion ( (like hydrogen)) might be an even better form of energy in the future, because there is very little radioactive waste left over – it involves smashing small atoms (like hydrogen) together to make bigger ones, but strangely (until you learn more physics!) the sum of the masses is again small – the missing mass has been converted into energy. The energy released is turned into heat, which heats a fluid, which turns turbines (like an engine in reverse) to make electricity.