An f-stop, or f-number, describes the size of the lens aperture. It is the focal length divided by the diameter of the aperture opening. A 50mm lens with a 25mm opening is at f/2. Higher numbers mean smaller openings. Each full stop, for example f/4 to f/5.6, halves the light.
Because it is a ratio, the same f-number gives about the same exposure with any lens. The word "stop" comes from early devices that stopped part of the light. One was the Waterhouse stop, a set of metal plates with holes of different sizes, named after John Waterhouse (1806-1879).
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