A barlow lens is an adapter that fits in between your telescope eyepiece and the eyepiece holder. This will increase the magnification by 2x 3x or even 5x depending on which one you purchase. For example, if you have a 10mm eyepiece giving 100x power , you will get 200x with the Barlow lens attached. Like with anything, you get what you pay for. Cheap plastic ones will do the job on a budget, but the results will be poor.
For better results go for glass lenses with metal barrels. Again there are so many to choose from. Single lens for those on a budget, and doublets/triplets for those who demand top results.
Most come in 1.25″ barrel size. However you can get 2″ versions.
Some will accept telescope filters for added vale.
Be aware not to push the magnification too much for your scope. If in doubt, consult an expert or go to your local club for advice.
Good quality barlow lenses can be purchased here http://shop.northernoptics.co.uk/telescope-accessories-118-c.asp