Question

what is nuclear fussion ?!

Answer 1

 Nuclear fusion is a reaction  in which two or more atomic nuclei come close enough to form one or more different atomic nuclei and subatomic particles (neutrons or protons). 

I hope this will help you

Answer 2

Nuclear fusion is the reaction in which two small nuclei combine to form a huge nucleus and a tremendous amount of energy is released.
Hydrogen bomb is prepared as a result of nuclear fusion.

Let me give you a fusion reaction.
See the above photo

Nuclear fusion cannot be controlled in any way. Thus it is not used in nuclear power plants to generate electricity, although it can produce more energy than nuclear fission.