nuclear fusion is much simpler. it's basically when a large nucleus is formed from two smaller atomic nuclei. and obviously energy is released when this takes place, as per usual. this process is used by the sun and much smaller stars, as it allows them to release energy. the actual sequence of nuclear fusion reactions is really complicated, but overall hydrogen nuclei fuse to form helium nuclei. so, for example:
hydrogen-1 nuclei fuse with hydrogen -2 nuclei to form helium-3 nuclei.
i'm pretty sure that's all you need to know, (i am retaking p2 this year but i haven't bothered to revise it yet, this is just what i remember), but you should definitely read about it in the revision guide and answer questions on the topic for further help.