Did you try wikipedia and not understand?
http://www.theanswerbank.co.uk/Science/Questio n540937.html
Potential energy is the energy stored in a system.
If I pick up a rock it has gravitational potential energy because if I drop it it falls and releases that potential energy.
Nuclear potential energy is the energy stored in atoms that can be released when a nuclear reaction occurs.
So for example one sort of Uranium has a tendancy to break apart, that is it's nucleus does. When it does energy is released (like the rock falling).
So you could say that the Uranium has nuclear potential energy that is released when the nucleus breaks apart (or decays)