"Imagine that you balanced a perfectly sharpened pencil on a tip occupied by one of these particles that spins left and right at the same moment. If the particle actually did move in both directions, the pencil wouldn't know whether to tip left or right. Or more specifically, it should tip in both directions at the same time. Now obviously, if you actually balanced the pencil in this way, you'd see the pencil tip in one of the two directions, because that's how reality works. What science hasn't been able to figure out is how reality chooses which of the two directions to make the pencil tip. The most interesting theory they've come up with states that reality doesn't choose, and instead branches off into separate parallel universes.
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If you take the many worlds theory of quantum physics to its logical conclusion and apply it to the thousands of tiny particles bouncing around in the human brain, and in every object you encounter on a daily basis (or any gun that gets fired at you), you get what's known as quantum immortality.

Basically, in any given situation in which it's theoretically conceivable that you survive, there is a timeline in which you actually survive. You're living in one of the infinite different versions of the world in which you survived. There are countless thousands of other universes in which you didn't survive, but you no longer exist in any of those. So the universe that you're aware of is one of an infinite number of universes in which you're just naturally, inexplicably luckier when it comes to not dying."