In philosophy, there is something called Possible World Semantics. A possible world is simply a logically possible description of the way reality could be. This world is the actual world. Here’s some examples of possible worlds:
1) A world where everything is the way it is in the actual world, but you are sitting a few inches to the left of where you’re actually sitting.
2) A world where only a frog exists.
3) A world where there’s unicorns.
These possible worlds don’t actually exist (though there are a few philosophers who seem to think they do), they are only used for thought experiments.
This is a tool used to talk about possibility, impossibility, and necessity. Something that is possible exists in some possible worlds, but not others. For example: dogs or unicorns. Dogs exist in the actual world, but there are possible worlds where they don’t exist. Unicorns don’t exist in the actual world, but there are possible worlds where they do. Something that is impossible doesn’t exist in any possible world. For example: square circles, or married bachelors. These things are incoherent because they involve contradictions, so it is impossible for them to exist. Something necessarily exists if it exists in all possible worlds. For example: 2+2=4 is true in every possible world. It’s impossible for 2+2 to not equal 4. Christians believe that God is a necessary being. He necessarily exists and He is necessarily good, so He exists and He’s good in every possible world.