As far as I can tell there really isn't lore. Someone just had an idea, "what if you noclipped through the wall and ended up in a strange universe where you were looking at the mirror images of the textures and you couldn't get back? And there are monsters and stuff. Wouldn't that be cool?" There are some memes about it but there really isn't much to it beyond that.
Oh no! Chromatic aberration!
Slenderman is just out of frame to the right. 2spoopy4me
Also they were like "woah, that photo of an empty room is really spooky for some reason!"