I only heard about the, "f/16 rule", a few years after I bought my first DSLR. Yes, that means I had made a ton of mistakes and ruined plenty of shots because I had no idea how to compensate for the contrasts I was having to deal with. It's a theory that basically governs the maths of the exposure equation and it revolves around midday on a sunny afternoon.
Even though there is a light meter built into your camera; it's not a fool-proof solution to every lighting situation. The f/16 rule is a good springboard because it illustrates the relationship between f/stop, ISO and shutter speed. It's a standard across all camera brands and models.
Have a look below at the ungraded images: the darker image was taken by using the light meter built into my camera. The camera was compensating for the light bouncing off the silver on the roof and utilised settings that it deemed adequate: f/16 1/125 ISO 100. For the lighter image; I used the f/16 rule: f/16 1/100 ISO 100. It was really bright all around so all I had to compensate was by lowering the shutter speed.
How does it work? Simply put: on a sunny day at midday set your aperture to f16. The result is that the reciprocal value of the ISO is the correct shutter speed. What? Ok, provided your f-stop is set to f/16; at ISO 100 your shutter speed will be 1/100 or at ISO 400 your shutter speed will be 1/400 (or very close to it).
Have a look below at the ungraded images: the darker image was taken using the automatic setting (Yes, I used auto for this). The camera was compensating for the light in the clouds and utilised settings that it deemed adequate: f/18 1/100 ISO 100. For the lighter image; I used the f/16 rule: f/6.3 1/100 ISO 100. Seeing as it wasn't sunny (nor midday) I compensated by lowering the ISO.
It's a notable rule because it can be applied anywhere in the world - a kind of universal constant that relates to the intensity of light coming from the sun. It's a reliable standard with two notable exceptions: snow (water) and sand (glass) both being exceptionally reflective. It's then the f22 rule.