An excerpt from wiki:
Schrödinger’s cat is an imaginary experiment — a thought experiment — devised by Erwin Schrödinger, which is often described as a paradox. It attempts to illustrate what he saw as the problems of the Copenhagen interpretation of quantum mechanics when it is applied to systems large enough to be seen with the naked eye, and not just to atomic or subatomic systems.
It is accepted that a subatomic particle can exist in a superposition of states, a combination of possible states. According to the Copenhagen Interpretation, the superposition only settles into a definite state upon observation. This is known as collapse or measurement.
Schrödinger proposed his “cat”, after a suggestion of Albert Einstein‘s. Schrödinger states that if a scenario existed where a cat’s state of life or death could be made dependent on the state of a subatomic particle, and also isolated from any possible observation, the state of the cat itself would be a quantum superposition — according to the Copenhagen interpretation, at least.
Schrödinger did not wish to promote the idea of dead-and-alive cats as a serious possibility. Rather he believed that those who would see the experiment as “absurd” had made a flawed assumption. Some, not realizing his cat is a demonstration of the strangeness of quantum mechanics, believe the cat is, indeed, half-dead and half-alive.
The thought experiment serves to illustrate the strangeness of quantum mechanics and the mathematics necessary to describe quantum states. It is widely accepted that particles can exist in a superposition of possible states. Large objects appear not to. When, how and whether any barrier is crossed between a microscopic quantum world of superposed particles and a macroscopic world of large, non-superposed objects is one of the major interpretational problems of quantum mechanics. Several interpretations of quantum mechanics have been put forward in an attempt to resolve the paradox. How they treat it is often used as a way of illustrating and comparing their particular features, strengths and weaknesses.
Well, I’m glad that’s all been resolved then. It’s all perfectly simple. Crystal clear. As mud.