ISO

The ISO is the sensitivity of the camera. The brighter it is outside, the lower the ISO needs to be. This is because the lower the ISO, the more sensitive to light the camera is and if it's very sunny or bright where the picture is being taken, the ISO doesn't need to be as high. If the picture is being taken in a low-lit area, the higher the ISO will need to be otherwise the image will come out as very dark and you may not even be able to see the image you desired. If the ISO is too high, the picture will be overexposed and will also have more noise. If the ISO is too low, the picture will be too dark and underexposed.