A supernova is the violent explosion that marks the end of certain stars’ lives. It happens in one of two main ways: a massive star runs out of fuel and its core collapses under its own gravity, or a dead star called a white dwarf pulls in enough extra material from a companion star to become unstable and detonate entirely.
For a brief period, a supernova can outshine every other star in its entire host galaxy combined, releasing more energy in a few weeks than the sun will produce over its entire ten billion year lifetime. The explosion blasts material outward at tens of thousands of kilometers per second.
Supernovae are also how the universe makes many of its heavier elements. Every atom of iron, calcium, or gold in your body was likely forged either inside a dying star or in the explosion itself, then scattered into space to eventually become part of new stars, planets, and eventually, us.