The easiest way to explain is with an example. If it’s a sunny day, your aperture is 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 take a picture 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 to get the same picture. 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. The rule works great when it's sunny.