Often you may have heard the term fast lens. A fast lens is the one which allow you to take photographs quickly. Quickly means that the time span for the entire shot is smaller.

For shot to be quicker, we should have Shutter speed set to a shorter value lets say 1/1000 or 1/500 etc. How does a fast lens helps in that?

A fast lens has a maximum aperture value to be something like f/2, f/1.8, f/1.4 which makes easier to use faster shutter speeds and make a quick shot.

For example a Nikkon 50mm f/1.8D is a fast lens as it has a very large aperture capability and thus under low light conditions one can use this aperture value to have faster shutter speed.

Nikon AF 50mm f/1.8

Fast lenses are usually expensive ones.