Basically it's related to the uncertainty principle; the cat is both alive and dead until you look inside the box.

Let's say i'm about to roll a dice. The result of that dice throw is in a state of superposition until it's thrown. Meaning that until it is thrown, all results exist. And it's only when the dice is thrown and the result is observed that the wave function collapses to a number from 1 thru 6.

From there, there are two camps of scientists:

One camp believes that all other superpositions dissipate once the roll is made. So if you roll a one, then the universes in which you rolled a 2 thru 6 just kind of evaporate, for example.

The Second camp that believes that all superpositions exist, and branch off into 6 different universes (one where each number was rolled) and they all continue unaware of the other universes existence (Multiverse theory)