I.S.O. indicates how sensitive an analogue film is to light. The lower the number, the finer the grain. It’s a measurement of sensitivity to light for digital cameras too: as the I.S.O. increases, so too does the sensitivity. So lower I.S.O. is appropriate for well-lit situations. Higher I.S.O. is appropriate in less well lit situations, when combined with a fast shutter speed.
But the higher the I.S.O., the more digital noise will be present. The images below, especially in the shadows, illustrate different noise levels at different I.S.O. levels.
This image is (relatively) low in noise (my camera’s not great) shot at 1/30th of a second at f 8.0, 38mm focal length, with an ISO of 125.
This second image is far higher in noise! It was shot at 1/8000th of a second at f 8.0, 38mm focal length, with an ISO of 25,600. The noise is the random variation in brightness or colour (for examples, the pink pixels) in the image.
Here is the ISO Contact Sheet with each the same image shot at each ISO level available for my camera.