It's a thought experiment. Here's a VERY brief rundown. Basically a dude puts a cat in a box for x amount of time. It is set up where at a completely random point in that time a poisonous gas can be released in the box and the cat could die or the gas isn't released and the cat survives. Now before the guy opens the box he does not know whether the cat is alive or dead to him the cat is both alive and dead, or in a superposition of states.