A barlow lens is a neat addition to your lens collection.
A Barlow lens is an extension. It is a lens that you place before your eyepiece. To be a bit technical it decreases the focal length of the lens to give a higher magnification. Well, all you really need to know is that a barlow lens increases the power of your eyepieces. This is really terrific because if you have two eyepieces, adding a barlow lens effectively gives you four eyepieces. And well you get the idea!
How do you use a barlow lens?
You remove the eyepiece from the telescope.
You then insert the barlow lens. And then, as the picture shows you insert the eyepiece into the barlow. That's it, you are done. Now the eyepiece has an increased magnification depending on what the barlow magnification is.
Buying a Barlow lens
Typically a barlow lens will magnify either 2x or 3X but there are also 2.5's and others. When buying a barlow it will be very clearly stated. If I were to give you a recommendation I would say go for the lowest power barlow available. (Maybe the 2X) This is because higher magnifications are not optimal with most telescopes. If you have a small telescope or an inexpensive one the higher power will just give blurryness or poor viewing (possibly).
Anyway the barlow is great because it doubles the number of lenses you have but just with buying one.
Another thing to think about is that telescopes and eyepieces come in different diameters! And you have to get a barlow that is the right size.
This picture shows two different size eyepieces. The diameter is different. The one on the left is a 1 inch eyepiece and on the right is a 1.25 inch eypiece. Just be sure to get a barlow that is the right size for your telescope/eyepieces.
Amazon.com has plenty of barlow lenses and from various manufacturers like Meade and Celestron:
I have sorted them out for you on amazon right here: Barlow Lenses