Nuclear Fission (concept used in nuclear energy)
Nuclear fission is a process where an atom 'split up' and release a huge amount of energy. The most common element used for this process is uranium. Uranium is labelled as unstable because it does not have strong enough bind force to hold the nucleus together.
In a similar way, the unstable atom is made to 'explode' by firing a neutron into the atom. As a result, the atom split up into two new atoms, a number of the neutrons and a lot energy. The energy released is millions of times more that the energy released through the combustion of petrol.
By using the extra neutrons released in the nuclear fission and firing into more unstable atoms, a chain reaction can be setup. This is the key principle behind nuclear power plant and nuclear bombs.