In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain.
Examples of ISO
Examples ISO
Best pictures
This is my best picture because it is the correct iso and isn't bleaches out. This image is also an example of Rule of Thirds. Furthermore, this image also shows the correct white balance. However, to improve this image I could focus on the aperture.
Worst picture
This is my worst picture because it is too bleached out and is over exposed. This image doesn't show the correct iso, as the camera is less sensitive to light.