Extension tubes are tubes that you place between your camera and objective. Extension tubes are used so you can focus from a smaller distance and thus you can get more of your subject on the photo.
Extension tubes have no glass. This results in no loss of quality, but you do lose light.
Extension tubes are often sold in sets of 3, mainly 12, 20 and 36 mm. The beauty of this is that you can decide what length your extension tube will be. You can use a single one or use any combination of the 3 together. The longer the extension tube, the bigger your subject will appear in front of your camera.
There are 2 kinds of extension tubes, with and without electrical contacts. Without electrical contact points the extension tubes are a lot cheaper, but you lose the ability of some of the automatic functions of the camera, like managing your aperture and auto focus. You can get these extension tubes for a few euro/dollar on the internet.
I prefer to work with extension tubes with electrical contact points because it let’s you use your normal camera functions. These are more pricey, think around 80 to 100 euro/dollar.
But the extra costs to me are worth it so you can still change the aperture and use autofocus.