Architectonic said it already, I will try to bring things into some context. I am no physicist so pardon incorrecntness.
Well first of all you have Heisenbergs uncertainity principle: you know that according to E = m * c² , every solid matter (m for mass, E for energy) is energy that got glued together. Then there is energy that hasnt become solid matter, like for example light. What light is made of, we dont know, but we describe its behaviour like that of a wave. If you use a sine wave-function you could describe the movement of light. So that means we have particles (matter) and waves.
The photon is a element of light. So what's a photon, a wave ? The thing is, depending on how you look at it, its either a wave or a particle. That's what Heisenberg was about. Heisenberg tried to capture a photon and when he did it was a particle. But when he didnt concentrate on a single photon but rather watched its movement, it behaved like a wave. That's called the Wave-particle-duality, which all particles inherit. For reference see Dr. Quantum ( Dr. Quantums Double Slit Experiment ).
Now three smart dicks, postulated the EPR-effect. That is the Einstein-Podolsky-Rosen effect. They said that they take a system with two particles which are one at the beginning. For example in an radioactive atom which is falling apart. Those two particles then leave the atom and travel in exactly opposite directions. They are quantenmechanically entagled. Now according to Heisenberg you can either only measure the place a particle is at or its impulse (movement). The impulse and the place can be shown as a vector. Like for example airplanes use it. A vectoir is a matrix of for example 4 numbers of which 3 are space coordinates and 1 is the impulse. Such a vector, applied to a quantum mechanical system is called the eigenstate. or eigenvector.
If we now dont measure neither the place nor the impulse of the particles, the result could be anything. Thats called the superposition of the eigenstate. In the moment of measurement, so when you take a picture, you'll only measure one eigenstate, namely the particle is next to Saturn or so.
So now back to the two particles, which are moving quantenmechanically entagled in the opposite direction. According to classical physic, I should be able to measure either place or impulse of both of the particles. But that is not the case. Fact is I can either measure the place of particle 1 and will get the place of particle 2 aswell or the impulse of both. So Heisenbergs theory now applies to two seemingly different particles. Instead of measuring place and impulse on could do the same with the particles spin but would get the same result.
This thought experiment was lately attested in reality by Niels Bohr I think to remember. It says that particles can be entangled in their quantum states and it says that something's very odd here and it's prolly our limited understanding so far why that is so.
Schrödinger back then now came and transfered this experiment into the macroscopic layer. He said if you put a cat into a box and put a radioactive nucleus into the box aswell, which will decompose but you do not know when. On decomposition it will trigger a geiger counter which then will smash a bottle with poison killing the cat. As long now as the box is closed, the cat is in superposition and can have the state of being dead or being alive. In the moment in which we oipen the box, the quantum mechanical system which the box resembles takes on one concrete eigenvector and therefore tells us where it is. That's the only way to find out if the cat is dead or alive.
Since thios is a game of probability you can say statistically that the cat is undead or dead-and-alive at the same time . Einstein said that this cant be all there is to it and commented it with the famous sentence "God doesnt play dice".