xx-F stop of Dynamic Range
The f-stop on a camera refers to its aperture setting. Aperture controls the diameter of the lens' diaphragm that lets light into the camera and onto the film or chip. The lower the f-stop number is, the more light that enters the camera. Because these numbers are based on ratios between aperture diameter and the focal length of film lenses, f-stop numbers are different for most digital cameras, but they can usually do about 14 stops. In comparison the human eye can do 18 to 20 stops (a very high Dynamic Range) and TVs 6 to 7.
By extension for HDR, the number of f-stops of an image is a way to describe very high contrast ratios using 2N. notation. For instance, if a camera is able to produce images with 10 f-stops, it means that the contrast value (ratio between white and black) can reach 2^14 (=1024). In other words, the white will be 1024 times brighter than the black.