Extension tubes are used for macro photography.
To make a lens focus closer than the designer originally intended, an extension tube can be fitted between the lens and the camera.
A lens focuses at infinity when the distance from the exit pupil of the lens to the film is equal to the focal length.
As the lens is moved away from the film, closer objects come into focus, but the amount of extension permitted by the focus barrel (the thing you turn) is limited at some point by a decision made by the lens manufacturer.
Extension tubes for SLR cameras usually have control linkages or electrical connections passing through them to retain any automatic functionality of the lens.