Right...well imagine your world. Everything you know. The moon, the sun, the stars.

Now imagine all of that is in a marble. Now imagine there's a marble just like it...but with a few differences. And the planet in that marble that's supposed to be your planet is the opposite of yours. The version of Ringabel in that world is a girl and the version of you is a boy.

In a multiverse there's a ton of similar but different planets.