The Sunny 16 Rule is a way to ensure correct exposure during daylight without using camera’s meter. On a sunny day set aperture to f/16 and shutter speed to the reciprocal of the ISO for a subject in direct sunlight. If you are shooting at ISO 200 then your shutter speed will be 1/200s or at ISO 800 then your shutter speed will be 1/800s. If you don’t have these exact shutter speed settings on your camera (i.e. you have 1/250s instead of 1/200) , just use the closest settings available. The sunny helps you to check if your camera is spot on with exposure or is it under or over exposed. The rule is also based on incident light ie it’s based on the brightness of the light only, and not how the light is being reflected off the subject and into the camera.