ISO measures the sensitivity that your camera has to light. It is useful to manually change the exposure on your camera when natural light levels are low or high. If there is not enough accessible light the ISO can be changed to let more light in however this can result in grainy pictures. We can use ISO to our advantage for example if we want our photos to be darker we can change it to suit the style. in this photo bellow we can see the effect that the ISO is having on the colors in the photo.  the number of pixels does not change however they are just more viable, we call this noise.