What is a Supernova Remnant ?

A supernova remnant is simply what is left over (i.e. the remnant) of a supernova. That probably isn't a very helpful definition, so I'll try to explain what I mean. First, I should probably define what is meant by the term "supernova." A supernova is simply the explosion of a star.

 The energy released in a typical supernova is on the order of 10⁴⁴ joules. (A joule is simply a unit of energy that physicists use). 10⁴⁴ is a 1 followed by 44 zeroes! There is no comparison for this amount of energy that is even meaningful. Supernovae are so powerful that they can be seen from halfway across the universe (billions of light years!). Here we can draw a comparison. The farthest star you can see with your eye on a dark night is only a few thousand light-years away.

What causes a supernova ?

Stars live out their lives by burning (via nuclear fusion reactions) light elements like hydrogen into heavier elements like helium in their core. For a star like the sun, this process will go on for about 10 billion years
before it runs out of fuel. More massive stars have more fuel to burn, but they go through it much more rapidly, so they actually live shorter lives. When a star runs out of hydrogen, it will try to burn helium into even heavier elements, like carbon, nitrogen, and oxygen. If those elements sound familiar, they should. You're a carbon-based lifeform, and you're breathing nitrogen and oxygen as we speak. All of those materials came from the core of some ancient star that exploded and spread its materials around the galaxy, before the Sun and the Earth were even formed!

What happens after a star goes supernova?

An explosion of that magnitude doesn't simply dissipate in a short amount of time (at least not by human standards). A supernova remnant is simply the expanding blast wave from the explosion plowing through outer space, as well as the remains of what was once the star following behind it.