Betelgeuse is classified as a red supergiant of spectral type M1-2  and is one of the largest stars visible to the naked eye. A supernova is the explosive death of a star in an event so violent that for a brief period that single star shines as brightly as a whole galaxy of more than a hundred billion ordinary stars like the Sun.  This is a relatively rare event.  When this happens, Betelgeuse will brighten enormously for a few weeks or months, perhaps becoming as bright as the full moon, and be visible in broad daylight.