It’s the energy produced by nuclear reactions.
Examples of famous nuclear reactions
Fission reaction
##quad_0^1 n + _92^235 U -> _56^141 Ba + _36^92 Kr + 3 _0^1 n##
Fusion reaction
##quad_1^2 H + _1^3 H -> _2^4 He + _0^1 n##Alpha decay
##quad_92^238 U -> _2^4 He + _90^234 Th##
Beta decay
##quad_6^14 C -> _7^14 N + electron + _0^0 barv##
We use fusion reactions to generate energy currently. In we may be using fission reactions to generate energy.