Marky, let me show you how google works ...

A Waggoner was once driving a heavy load along a very muddy way. At last he came to a part of the road where the wheels sank half-way into the mire, and the more the horses pulled, the deeper sank the wheels. So the Waggoner threw down his whip, and knelt down and prayed to Hercules the Strong. “O Hercules, help me in this my hour of distress,” quoth he. But Hercules appeared to him, and said:

“Tut, man, don’t sprawl there. Get up and put your shoulder to the wheel.
“The gods help them that help themselves.”

You might recognize this as Aesop's work, one of your contemporaries?

Our OP asked a legitimate question. But instead of spending the 0.55 seconds it took Google to generate about 12,100,000 results and to then read and learn and develop their own knowledge ... they did ah .... what? Asked others to google and read it for them ...

And how many of the god-worthy UHH KXers actually did something else than the exact same google and then just cut n paste here? More than one chart was copied directly from the image results from Google into responses to this post ...

Aesop, see reference above, is thought to have lived in Greece about 2,600 years ago, so nothing new here although no one in no time ever before has had access to 12,100,000 pieces of knowledge available in just 0.55 seconds ...

BTW - hopefully those following this chit-chat worthy discussion have learned too that diffraction issues in digital sensors may have moved the sunny 16 at f/16 to the dust heap of photography best practices ...

I means that on a sunny day you can bet from 10 AM to 2 PM your exposure should be 1/125 sec. at f16 at ISO 100.

The sunny 16 rule works like this:

On a clear and sunny day, at an aperture of F/16, you will get a correct exposure if you use a shutter speed that’s the inverse of the ISO speed you’re using.
The second part is probably the one that’s confusing you (if any of it is). You have to know what ISO speed is in order to decipher what’s going on. So allow me to explain.
The easiest way to explain is with an example. If it's a sunny day, and have your aperture set to F/16 and ISO set to 200, to correctly expose your image the shutter speed needs to be set to 1/200 (the inverse of the ISO number).

ISO speed is your camera’s sensitivity to light. A bigger ISO speed means a larger sensitivity. If your camera is more sensitive to light, it takes less light to make a picture more bright. Most cameras start out at an ISO speed of 100, and some models go as high as ISO 1600. That’s 16 times more sensitive than the default, meaning you’d need to expose the camera to 16 times less light in order to get the same picture. I talk more about ISO in ISO Explained.

Keep this in mind because you’ll notice a pattern. The rule says you need to use the inverse of the ISO speed. That’s interesting because as you increase your ISO speed, you effectively have to increase your shutter speed to compensate. At ISO 200, your camera is twice as sensitive to light, so you need to use a shutter speed of 1/200 of a second to let in less light and balance it out.

Let’s use another example. Let’s say it’s a sunny day, and your camera is set to ISO 400. According to the sunny 16 rule, if you use an aperture of F/16 and a shutter speed of 1/400 s, you will have an evenly balanced image that is neither too bright nor too dark.

That’s interesting, but it seems like the rule can only help us out when it’s sunny.