What are Extension Tubes?
They're a tube that fits between the lens and camera body to help with macro photography.
its sole purpose is to move the lens farther from the image plane. The farther away the lens is, the closer the focus, the greater the magnification
There is also loss of light so your aperture and/or exposure time will be affected.
You can buy ones specific to your camera that have electrical contacts that allow autofocusing etc to work, or generic ones that have no contacts and therefore force you to use manual focus.