A supernova is what happens when a very large star known as a red giant, many times more massive than our sun, runs out of its nuclear fuel and suddenly cools down, causing it to collapse onto itself and explode. When a red giant star in our Milky Way Galaxy goes supernova, it creates, for a few months, a light brighter than anything in the night sky other than the moon. This has happened three times in the last 1000 years, in 1604, 1054, and 1006. The last two helped advance the cause of science by proving that the stars in the sky were not "fixed."
Betelgeuse, a very bright star in the constellation, Orion, could be giving signs of going supernova. It has been measurably dimming for several months which is possibly due to the red giant approaching its cooling-off phase. Astronomers are uncertain if or when it will go supernova, but it would certainly be the most spectacular stellar event of our lives if it does.