I consider a set of extension tubes essential if you like macro photography. In addition, you can use them for other types of work such as bird photography and even portraits. Let me explain.

Extension tubes are basically spacers. They come in a set of three, such as the Kenko tubes pictured below, and they fit between the lens and the camera body. They don't have any glass, so the quality of your prime lens isn't affected in any way. They can be used with a macro lens, such as a 50mm or 100mm macro, or they can be used with a telephoto lens such as a 70-200mm zoom or even a 500mm super telephoto. Their primary function is to enable you to focus closer, to fill the frame with very small subjects.

When you place one or more tubes between the body and a telephoto lens, the minimum focusing distance is reduced. An example is the photo of a red bellied woodpecker, below. This bird visited the feeder outside my office window, and it was only 10 feet from my desk. I wanted to use a 500mm f/4 to fill as much of the frame as possible with the bird (this photo has not been cropped), but the minimum focusing distance of this lens is about 15 feet. By placing an extension tube between the lens and the body, I was able to focus to 10 feet.   

This picture was taken right through the glass window. 

Do extension tubes cause a loss of light? Yes, they do, but only when you use them for closeup photography with a short focal length lens. They don't cause appreciable light loss when used with a telephoto. When using them with lenses such as a 50mm or 100mm macro, a significant amount of light is lost. The amount of loss depends on how much extension you use. All three tubes used on a 50mm macro cause a one full f/stop loss. The same loss is experienced, though, in any closeup work with any type of lens or extension.  

For example, Canon has a 5x 65mm macro lens, and when you zoom in from 1x to 5x, you lose light. This is just the nature of light as it travels away from the source. Since you should be using a tripod when doing macro work, the loss of light isn't a problem unless your subjects are moving. In that case, you'll need to use flash to freeze the movement.

Many photographers who enjoy doing macro photography prefer a telephoto macro, such as a 180mm macro, as opposed to a 50mm macro lens. Telephoto macros allow you to focus on small subjects from several feet away, instead of several inches, and the background goes out of focus more readily. Completely undefined backgrounds are usually more desirable in closeup work because that forces all of the attention on the subject as you can see in the picture of the caterpillar.

If you use extension tubes on a medium telephoto, you instantly have a telephoto macro. That's why I often use an extension tube(s) in combination with my 70-200mm f/2.8 lens. This gives me a telephoto zoom macro!

The greater the magnification, the more depth of field is lost.  That means that when you use extension tubes, you may not be happy with intricate and fascinating subjects appearing largely blurred.  This is why I often use a flash when doing macro work.  The additional light from the flash allows the use of small lens apertures like f/22 or f/32 which I feel are necessary for most closeup work.  The photos above of the orchid, the blue dart frog, and the caterpillar were all taken with a ring flash and a lens aperture of f/32.