Sunny f/16 Rule is?
Basically, it is that for correct exposure from 1 hour after sunrise to 1 hour before sunset, on a clear, SUNNY day and when using an aperture of f/16, the shutter speed denominator is the same as your ISO setting. Want to check it out (and make sure your in camera meter is reading accurate)? Late morning, point your camera to a big patch of blue sky (so it fills your viewfinder) in the west (opposite in the afternoon) in Aperture-Priority mode, set at f/16, with no filters on you lens. Your in camera meter should display a shutter speed within about 1/3 of a step of 1/200 second if your ISO is set to 200, 1/400 second, if your ISO is 400, etc. If it's not close, you may want to have your meter checked.
With all the cool gadgets on our cameras, its easy to forget these helpful basics of exposure.