The f-stop numbers are primarily a system that represents the amount of light reaching the film during exposure. The smaller the number, the more light reaching the film.
Photographers use the f-stop numbers to control exposure.
The meaning of the word photographer is one who controls the light. Learning and using f-stops is the most important method of contolling and understanding light. The aperture is the opening in the diaphragm of the lens that lets in light. The f-stops represent how much light is able to pass through the opening. As the aperture ring is turned, the opening closes or opens, allowing varying degrees of light to pass through the lens.