As promised, the following is an example of what extension tubes do. I'll leave the more detailed description to the makers of the tubes themselves, Kenko, but essentially the extension tubes allow a camera's lens to focus closer to it's subject. This has the effect of magnifying the image being taken.
The extension tube set that I bought has 3 different tubes; a 12 mm, a 25 mm and a 35mm. The lens that I've us for this example is a Canon 50 mm, f1.8.
This first shot (of a pin cushion) uses no extension tubes, just the lens. I focused as closely as possible on the pin cushion.
The next shot used only the 12 mm extension tube and was focused as close as possible:
This one used the 12 mm combined with the 25 mm extension tubes, focused as close as possible: