Lens speed indicates the aperture of a lens. A fast lens has an aperture of f/1.4. The lower the f-number, the bigger the aperture, and this are useful for taking photos in places that are dimly lit, like the inside of a cathedral. All lenses are at least capable of about f/4 and f/5. It is below f/2 that lenses are best suitable for perfect low light photography. Of course, if you are exclusively going to perform sunlight shots, a fast lens is not needed.