Explain what f stop means

In digital photography and digital video recording, F-stop refers to the international standard sequence of numbers that express relative aperture. F-stop is the "lens focal length" divided by the "effective aperture diameter." The smaller the F- number, the greater the amount of light that passes through the lens. Each change of F-stop halves or doubles the image brightness as you step up or down.

Hope this helps. Susu

In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens; in simpler terms, the f-number is the focal length divided by the aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.