The Sunny f/16th Rule is a way to meter for correct exposure during daylight without having to rely on the cameras meter. For example, if your ISO is 800 at f/16 your shutter speed will be 1/800sec. If your ISO is 100 at f/16 your shutter speed will be 1/100sec.
This rule has helped me through some interesting times in rainforest, jungles, Mt. Everest etc.. At such times when my cameras meter battery gave out, I have even owned a few cameras with no meter. Of course those days are all but gone now, I still have my mechanical cameras, they are semi retired,  sitting in display cases.  I also have over 100 rolls of film in my refrigerator, using from time to time:):):)

Happy Zen Photography