Many photographers use extension tubes to turn their lenses into macro lenses. An extension tube is just a spacer that sits between the lens and the camera body to alter the MFD (minimum focus distance). The MFD of a lens is a measurement of the closest point a subject can be from the camera’s sensor, whilst still being able to focus. If a subject is closer to the sensor than a lens’ MFD, you cannot focus on that subject.
The thicker the extension tube used on a lens, the higher the lens magnification becomes as the photographer is able to move the lens closer to the subject and still achieve focus. I tend to use a Helicoid rather than tubes as I can then change the minimum focal length at will.
See Extension Tubes HERE