This tube fits between the lens and the camera body and allows for closer focusing by extending the lens. The 17mm is the 2nd thinnest tube available. I sometimes will use this on portrait lenses to get in very tight, and also on my macro lenses.
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 one new one left in inventory in original sealed box. Price greatly reduced from original list of over $500