The amount of light hitting the film is called the exposure. To get a photograph that is not too dark or not too light the photographer uses the f-stops and shutter speeds to control the amount of light hitting the film.

Image result for camera exposure dial
Under Exposed Photo
Manual Exposure. Photo by Robin Mehdee

The photographer determines how much light is needed by using a light meter, by viewing the display/screen , or from experience. The photographer can set the f-stop and then move the shutter speed dial until the light indicates the correct exposure. Modern cameras generally have a setting that will automatically set a proper shutter speed and f-stop. Also a dedicated exposure dial.