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.
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.