In traditional (film) photography ISO (or ASA) was the indication of how sensitive a film was to light. It was measured in numbers (you’ve probably seen them on films – 100, 200, 400, 800 etc). The lower the number the lower the sensitivity of the film and the finer the grain in the shots you’re taking.Higher numbers mean your sensor becomes more sensitive to light which allows you to use your camera in darker situations. The cost of doing so is more grain (although cameras are improving all the time and today many are able to use high ISO settings and still get very use able images). 100 ISO is generally accepted as a ‘normal’ or ‘standard’ ISO and will give you lovely crisp shots (with little to no noise/grain). Most people tend to keep their digital cameras in ‘Auto Mode’ where the camera selects the appropriate ISO setting depending upon the conditions you’re shooting in (it will try to keep it as low as possible) but most cameras also give you the opportunity to select your own ISO also. (information taken from the internet.)
My examples of ISO
I have changed the ISO the first picture
Auto 100 200
800 1600 3200
worst
This is the worst because the iso is really dark.
best
This is the best because you can see everything clearly.