Penumbra's atmosphere and sound design

[The chaser]

Does anybody know what the first note means ("Between light and shadow, exists an entire world. That world is necessarily our own. Get out. Now. O.")?

I don't know much about the "O" thing, but I guess it talks about the mines. The world of the infected. The world of the Tuurngait. You are a stranger, they dislike you. I think it's that, facepalm me if I'm wrong.