Why is iron significant to understanding how a supernova occurs?

iron is the heaviest of all atomic nuclei, and thus no heavier elements can be made.
the fusion of iron into uranium is the reaction that drives a supernova explosion.
iron cannot release energy either by fission or fusion.
supernovae often leave behind neutron stars, which are made mostly of iron.

Iron cannot release energy either by fission or fusion.

Explanation:

When a high-mass star runs out of hydrogen inside its core, it starts burning helium, and then heavier elements until it reaches iron.

Nuclear fusion of iron cannot occur, because the amount of energy required for the fusion of two nuclei of iron is larger than the energy released.

Therefore, when all the lighter elements lighter than iron have already fuse together, the star cannot burn any other element, so the nuclear fusion inside the core stops and the star collapses on itself, producing a huge explosion known as supernova.