ISO values were originally writen on the outside of film canesters, to show how reactive to light that type of film was.
| ISO100 film:
|| ISO1600 film:
If you knew you would be taking photographs outside in the sun (on holiday?), then you would buy ISO100.
But if you expected to take photos indoors, or of fast moving objects (race cars?), then you might buy ISO1600 film.
Modern cameras don't use film anymore, instead there is an imaging sensor.
Each pixel on the sensor detects how much light has hit it, but before this infomation is read by the camera, it gets amplified.
So the ISO setting on a modern camera is actually how much amplification is done on the sensor, before the information gets read by the camera.
The higher the ISO value, the more amplification of the signals, the more exposed the final image is.
ISO has other effects beyond exposure, the most significant being noise.
The more you amplify a signal, the more noisy that signal becomes.
You can experience this yourself by turning up a clock radio to maximum volume.
What you will hear will not simply be a louder version of what it sounds like normally, it will most probably sound terrible.
|low ISO, low noise:
||high ISO, high noise: