Before Everett and his concept of multiverses came along, physicists were stuck in a bedeviling bind. They had to use one set of rules for the subatomic world that quantum mechanics focuses upon, and another set of rules for the large-scale, everyday world that we can see and experience. The complexities of this shift in scale forced them to twist their brains into some strange shapes.
For example, in quantum mechanics, particles don't have set properties when no one's looking at them. Instead, their nature is described by something called a wave function, which includes all of the different possible properties that a particle could have. But in a single universe, all those possibilities can't exist at once, so when you look at a particle, it settles upon one single state. That idea is illustrated metaphorically by the famous Schrödinger's cat paradox — that is, if there's a cat in a box, it's both alive and dead at the same time, until you open the box to check. Your action forces the cat to become either a warm, breathing kitty or carcass.
But in a multiverse, you don't have to fret that you're going to kill the cat with your curiosity. Instead, whenever you open the box, reality splits into two versions. Sure, there's one realm in which you're thinking, "Ewww!" as you gaze at a dead cat. But there's another version of the event in which you might be scratching its ears as it purrs [source: Merali].