Photographers set their exposure using a combination of shutter speeds and f-stops to get the correct amount of light on the film or digital sensor. The f-stop regulates how much light is allowed through the lens by varying the area of the hole the light comes through. The shutter speed regulates how long the film is exposed to light coming through the lens. For any given film speed (ISO), there is one correct amount of light to get a properly exposed photograph.
In the following video, Dylan Bennett explains focal length and lens diameters and how it relates to the f-stop scale. It’s a great explanation of f-stop values to help you understand how to double or half the amount of light you take into your camera. I particularly like the second half of the video when he explains how to remember the f-stop scale of numbers. Check it out!