The above chart is based on the sunny 16 rule. The rule being that on a sunny day with an f/stop of f/16 the ISO and shutter speed will be of like values. In the film days the ISO was predetermined by the film. So film with an ISO of 100 you would set the shutter speed at 1/100 or 1/125 of second. Beyond that the above chart shows many equivalent exposures based on changing the ISO and f/stop.