Sunny 16 is a light metering guideline that says proper exposure on a sunny day is f/16 at a shutter speed that is the reciprocal of your film speed. The exposure is reduced by one stop for sun+clouds, another for cloudy and so forth, e.g.:
For example, for a film speed of 100 ISO/ASA, a speed of 1/125 would be selected. For 200 ISO/ASA, 1/200 or 1/250, and so on. The closest match to the speed of the film to shutter speed being desired, depending on the speeds available.
Weather Symbols Edit
Sunny 16 is sometimes formalised onto a camera as weather symbols. The shutter speed and aperture controls are marked - one with the film speed and the other with sunny/cloudy (etc) symbols.