In the days of film cameras, once you insert a roll of film (24 or 36 frames) you are stuck with it. This means you are locked in to the ISO. If you inserted ISO100 you will be good for bright day time scenes but not night and indoor shots (unless you use a flash).
But this changed with digital cameras. With digital sensors you can change your ISO setting and even have automatic ISO. In film cameras you choose an aperture and the shutter adjusts to match or choose the shutter speed and the aperture adjusts to match.
But with digital cameras you can set both the aperture and shutter speed and the ISO will adjust to make the correct exposure. This difference is a significant improvement on shooting workflow.
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