ISO is the level of sensitivity of your camera to available light. The lower the ISO number, the less sensitive it is to the light, while a higher ISO number increases the sensitivity of your camera. The component within your camera that can change sensitivity is called a sensor. It is the most important part of a camera and it is responsible for gathering light and transforming it into an image. With increased sensitivity, your camera sensor can capture images in low-light environments without having to use a flash. But higher sensitivity comes at a cost – it adds grain or “noise” to the pictures.
The lower the ISO number on the sensor can produce the highest image quality, without adding noise to the picture. My Nikon D610 is 100, while most Canon digital cameras have a ISO of 100, not all camera’s can get that low, but can get within reason. So, optimally, you should always try to stick to the lowest ISO to get the highest image quality. However, it is not always possible to do so, especially when working in low-light conditions.