A supernova is a star that suddenly gets a lot brighter from an explosion that ejects most of its mass. In other words, it's the end of a star.
An example of a supernova remnant(SNR for short) is the Crab Nebula. It's called that because, well, it looks like a crab, duh. Looks nice though.
An amateur astronomer in Japan found a mention of a bright star in the sky. It was written that it was seen around 1054. Its position matched perfectly with the Crab Nebula, and so scientists now know for a fact that the Crab Nebula was the remain of a supernova in 1054.