Verified answer

It's basically showing that a random, unpredictable event (in this case, radioactive decay) can cause an object unrelated to the first object to change. In this case, the changing object is the cat.

By pure luck, the radioactive substance could start to decay. This sets off a chain of events that eventually wind up with a jar of poison being broken. The cat then drinks the poison, and dies. The cat is in a confined space so no outside events could affect it.

These different events are known as "quantum events", which are part of the quantum theory. It's saying that the cat can be both dead and alive, and that the radioactive substance can both be decaying and not decaying. To account for these events happening simultaneously, there must be a different universe in which each of them can take place. It's part of the parallel universe theory, and an example of quantum mechanics.

I like to explain it like this: Flip a coin inside a box, then close the lid whilst it is still spinning (i.e. you do not know which way it lands until you open the box and have a look). Until you open the box, is it heads or tails?

The answer is neither! Until you have a look (whereby you force it to be either heads or tails) it exists in all possible states. It "collapses" into one state when you observe it.

The other interpretation is like this: once you open the box to have a look, the universe splits into two different universes: in the first one, it's heads. In the second one, it's tails.

It is a lot harder to explain why this is the case.

No, no simpler way to explain it but there are ways far more complicated

It is a little confusing for non physicists. Keep reading that wikipedia page until you get it