Stars generate energy in their cores, fusing lighter elements into heavier ones. This is how a star prevents its own gravity from making it collapse; the heat generated inflates the star, creating pressure that holds it up.
The most massive stars take this energy production process to the extreme; while lower mass stars like the Sun stop after fusing helium into carbon and oxygen, massive stars continue on, fusing elements all the way up to iron.
However, once a mighty star's core is iron, a series of events takes place that actually removes energy from the core, allowing gravity to dominate. The core collapses, setting up a huge blast of energy that is so immense it blows away the outer layers of the star, creating an explosion we call a supernova.