Why is iron significant to understanding how a supernova occurs?

Answer;

Iron cannot release energy by fusion.

Explanation; Supernovae are some of the most magnificent events in the cosmos. When a massive star dies, it can result in a spectacular explosion many times that of man's total nuclear arsenal. When a star has used up all the lighter elements and has just iron left, it has no more nuclear "fuel". That causes the star to contract then explode very violently as a supernova.