Photographers set their exposure using a combination of shutter speeds and f/stops to get the correct amount of light on the film (or sensor--this all works for digital too). The shutter speed regulates how long the film is exposed to light coming through the lens. The f/stop regulates how much light is allowed through the lens by varying the area of the hole the light comes through. For any given film speed and lighting combination there is one correct amount of light to properly expose the film. This amount of light can be achieved with many different combinations of f/stops and shutter speeds. This page goes over the f/stop and especially its initially-confusing numbering at some length.