A supernova is a big explosion when a star explodes.
A supernova blasts away heavy elements (silicon oxygen nitrogen iron lithium etc.) produced in the star for hundreds of light years. Stars that have more mass than continue fusing heavy elements until it is time to fuse iron. Iron is such an heavy element that the star cannot fuse it. In other words the star is collapsing and its entire mass is being pumped into the core. The core then collapses and depending on the mass of the star it either becomes a white dwarf a neutron star or a black hole. When the core collapses the star explodes and this is how a supernova happens.