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.
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.