F/stop

The f/stop regulates how much light is allowed through the lens by varying the area of the hole the light comes through.
F-stop or the maximum aperture of the lens. This figure is derived from dividing the focal length of the lens by the aperture opening of the lens.
The aperture is the opening formed by a system of metal leaves in the lens that open up and close down to control the volume of light passing through the lens. It is the lens's equivalent of the iris of our eye.
The photography term we often see in photo magazine "opening up 1 F-stop," means making the aperture size larger to allow more light through the lens and "stopping down" means making the aperture size or F-stop smaller to allow less light through the lens. A larger aperture size is represented with smaller number hence f2.8 is larger f-stop than f5.6.

Reference

Cole.M (14th May 2011) A Tedious Explanation of the f/stop. Available from: http://www.uscoles.com/fstop.htm [date accessed: 10th October 2011]

STsite.com (2011) F-Stop and Aperture. Available from: http://stsite.com/camera/cam04.php [date accessed: 10th October 2011]