They're similar to extension tubes in that they move the lens further away from the camera body to allow a dramatic decrease in the minimum focus distance which in turn allows you to take extremely close-up photographs!
You can get dedicated macro lenses but I've got a set of extension tubes that I love.
The IQ isn't affected as there's nothing but air inside them, no lenses or glass. The only thing to be aware of is that once you get very close to a subject there is often a lack of light. The other thing is that the depth-of-field with these is also dramatically smaller so it can be a challenge to get enough of your subject in focus while still allowing enought light through.