Just to be even clearer on the extension tube issue...
You'll note that when you focus a lens it gets longer and shorter. When it is at infinity it is at its shortest--rotating the focus ring doesn't move it any farther in. Likewise, when you focus closer, rotating the focus ring makes the lens get bigger--it sticks out more. If you do this, back and forth, with the lens off the camera you'll see that all that is happening is that the entire lens assembly is just moving toward and away from the film--the lens barrel just provides a hollow tube. Now, focus the lens all the way out, focus it as close at it will go. At some point the focus ring will stop rotating and you can't get the lens assembly any farther from the film. That's the closest the lens will focus--without extension tubes.
But note that what is stopping you from focusing closer isn't the optics, but the lens barrel itself. But since the barrel is just a hollow tube, what if there was a way to make the tube just a little bit longer--to move the lens a tad farther away and to thus focus even closer?
That's what extension tubes do. They add a little bit of length to that hollow tube called a lens barrel (which is why extension tubes themselves are nothing more than hollow tubes without any g). They come in different sizes which you can combine in any way you want because once you start doing this you'll start getting fancy about it. The bigger the tube you add the closer the lens will focus (since the glass keeps getting farther and farther from the film).
Note: Depth-of-field? Very slim. Exposure adjustment? With a prism looking through the lens the adjustment is automatic. With a handheld meter you have to tell the meter that you added tubes --the glass is farther away down that hollow tube called a lens barrel and/or extension tube so the light is dimmer, right? In fact, it was dimmer even without the extension tubes, as you focused closer and the glass got farther from the film, but we all just sort of ignore that most times