The manila room is a room in Level 0, it doen't contain much aside from a table, a chair and a clickable note. The note reads:
"If you are reading this you have entered the manila room, if you want to escape this level you have to No-Clip through a green thing on the ground. Go trough the blue thing to leave this room. "
-From the M.E.G
You can enter via a special hallway in Level 0 and you can exit via a blue blob that takes you back to Level 0.
Gallery:
Manilla room, note shown
Manilla room, note hidden