A Barlow lens is the astronomy accessory that keeps on giving! Insert it between your eyepiece and your telescope to get double the magnification instantly. Let’s say you have two eyepieces in your accessory case, a 10mm and a 25mm. Add a Celestron Omni 2X Barlow Lens to the mix, and now you have four magnifications to choose from. Combining this lens with the 25mm provides double the magnification, or the equivalent of a 12.5mm eyepiece, while the 10mm gives you the power of a 5mm eyepiece. 
Eye relief is the distance your eye has to be from the first lens in the eyepiece in order to see the full field of view. When you use a Barlow lens with an eyepiece, you get double the magnification, but the eye relief is not affected. This is a good thing, because normally the shorter the focal length of the eyepiece, the less eye relief you have. In other words, a standard 10mm eyepiece, as a general rule, will have better eye relief than a 5mm eyepiece. Bonus! 
The Celestron Omni 2x Barlow comes in a sleek, silver package. The two-element optics are fully multi-coated, and the edges of the lenses are blackened to cut down on internal reflections. The 1.25” barrel will fit in standard 1.25” focusers and diagonals, and the Barlow will accept 1.25” eyepieces. A metal thumbscrew locks the eyepiece firmly in place.