Well, I dunno. I seem to remember reading about this somewhere on my travails. It is an invisible force field that acts like a solid brush, meaning it will block player movement and stuff, and you put it around decorative objects like window panes and trees and shrubs. (and stuff)
This way, if you make stuff out of sheets or any other object that wouldn't block the player from moving into it, it will then become an interactive element so the player can't for example, walk through a tree like it wasn't even there.