The letters ISO on your camera phones refer to the film speed. Even though your camera phone is most likely not film at all, but rather digital, the ISO setting still does has the same function as film cameras. ISO determines how sensitive the image sensor is to light. The higher the ISO speed (higher ISO number), the higher the sensitivity to light is increased. This will allow you to shoot images in dark conditions without flash, creating more natural-looking night-time pictures.

Increasing the ISO speed also brings its disadvantages. As the ISO value is increased it increases its sensitivity to light making full use of the available light, but at the same time the unwanted image noise is increased. This noise can be seen when you zoom into a picture and you would actually see small grains in the image.