What is supernova remnant?

When a star supernovas, it blows away a significant fraction of its mass. The expanding ejected layers can be seen as the supernova remnant.
One such remnant is the Crab Nebula.
Sometimes, the expanding remnant sweeps up material in front of it, like a snowplow, and even collides with a cold molecular cloud, causing it to fragment and collapse. In this way, it is possible that stellar death can actually lead to stellar birth.