A Barlow lens is a focal multiplier. It increases the focal length of a telescope or a camera lens by a certain factor (usually 2x, 3x or 5x). Optically, it also reduces the widest aperture by the same factor.
For example, if you add a 2x Barlow lens to a telescope with a focal length of 500mm and an aperture of f/6, you will obtain a focal length of 1000mm and an aperture of f/12.
In conventional photography, these are called tele-converters, and are similarly attached between the lens and the camera to provide a greater focal length (usually x1.4, x1.7 or x2).