This makes the already slightly unstable heavy atom even more unstable, which causes it to split into smaller atoms and possibly eject a few neutrons.
This releases a lot of energy because the original atom was so unstable.
Usually the 'pieces' are smaller atoms, but often neutrons are also released.
Many fission reactions are initiated by a very heavy atom being hit (or, in chemistry lingo, bombarded) by a neutron.
Currently, there is no feasible way to harness this energy from fusion and use it to power our infrastructure, but several years down the road, it is possible that it could supply us with the energy we need.
In a way, we are able to use some of the energy from fusion reactions.
Whenever nuclear power is discussed, whether it be in the form of a nuclear reactor or a nuclear-powered submarine, the energy is produced because of a fission reaction.