This tube fits between the lens and the camera body and allows for closer focusing by extending the lens. The 9mm is the thinest tube available. I use this one on my portrait lenses to be able to fill a little bit more of the frame with the subject.
9mm, 17mm, 34mm, and 67mm tubes are offered plus a vario extension tube with 22-68mm range and the extension bellows which can extend from 67mm to 204mm.
Note: I have two of these in stock in original sealed boxes - price lowered greatly from original list price of over $500