just read the ebay page "This macro extension tube set is suitable for all Canon cameras. Transform your Lens into a Macro Lens. It moves the lens from the film or digital sensor. The closer the focus is, the greater the magnification. The extension tube does not affect image quality as there is no optics inside. In comparison, a close up filter is often blamed for degrading the quality of image. Auto-focus function cannot operate.
This tube set consists of 3 individual tubes, camera body mount and a lens adapter. The 3 individual rings can be used separately with the camera body mount and a lens adapter, and of course the magnification ratio will be different."
Basically extension tubes move the glass elements further away from the sensor and allows the lens to focus much closer than it originally could (if you did physics it would help - same principle that is used in glasses that help people with short/long sightedness)