ISO defines the measure of sensitiveness to light by the camera lens. As ISO gets lower and light becomes more. Therefore, for instance you are doing your photo shooting from outside when it’s sunny, you are supposed to shot at low ISO like 100. With such setting, you will get a crisp, clear image. If working from an enclosed place where light is low, adjust the ISO such that the sensor gets more light.
Always remember that cameras are quite different and therefore you need to practice using your own camera such that you note the ISO points which work best for your camera. However, remember that higher ISO is always accompanied by a cost with high ISO resulting in grainy pictures and therefore, work so much to maintain the ISO as low as possible.