Focal length divided by the aperture diameter. In full manual mode everything is fully adjustable with each setting being as equal important as the other: shutter, f/stop, ISO, etc..these are all things that could be learned with a point & shoot before jumping into a DSLROriginally Posted by 13Bracer
http://en.wikipedia.org/wiki/F-number
The following images are the same shot with different f/stops, the first is f/5 and the 2nd is f/32, two completely different looking photos but the same shot with only a couple settings changed...