Extension tubes are hollow cylinders without optics designed to enable a camera's lens to focus closer than its minimum focusing distance and therefore provide more magnification. They are placed between the lens and body and are particularly useful in allowing lenses without macro capability to shoot macro, but in general, they offer increased magnification for whatever need. 

An extension tube increases lens magnification by an amount equal to the extension distance divided by the lens focal length. For example, adding a 25 mm extension tube to a 50 mm lens will give a magnification gain of 0.5X. Therefore, if the lens's original magnification was 0.15X, then the new magnification will be 0.15X + 0.5X = 0.65X.

Kenko's Auto Focus extension tubes are designed with all the circuitry and mechanical coupling to maintain auto focus and TTL auto exposure with Nikon Z lenses.