When you heat something up to high enough temperatures, it glows. Think of a heating element on a stove, or a piece of steel in a forge. As the temperature goes up, it changes from a dull red to a bluish-white as it gets hotter. If you could continue to heat it without melting it, it would eventually get so hot that the light would come primarily in the form of X-rays. In order to see this extremely hot gas, you have to observe it with X-ray telescopes.
Supernova explosions are among the largest explosions in the universe.
The atoms in your body, in the chair you are sitting in, and in the Earth we live on, were produced by nuclear fusion in the interior of stars, and were dispersed into the interstellar medium in supernova explosions. In the process of mixing with the interstellar medium, a supernova remnant is created. The youngest supernova remnants that we can observe today are about 400 years old, and were created by three supernova explosions that occurred around the turn of the 17th century. There had not been another nearby, bright supernova explosion since then, until 1987. This Chandra observation of SN1987A lets us observe the actual birth of a supernova remnant for the first time, and lets us test our theoretical models against observational data.