ISO, stands for International Standards Organization and it is a standardized industry scale for measuring sensitivity to light. This affects the sensor in the digital camera. When it comes to ISO the higher the level the more grain or noise you will get in your shots, compared to a lower level where it will cause your shots to be less grainy, as seen below.
Lower Iso tends to create a more dynamic image, by being able to contain all the important colors and textures. Most uneducated photographers will shoot on auto and have no idea how ISO is altering their photos. More skilled photographers will realize what setting they are in and adjust accordingly to accommodate for the surrounding area.
Many things can affect your ISO level that you will set like
-Lamps and natural light sources
Sometimes you might have to bump up your ISO a bit. To keep fast moving things like runners from being blurry, use a fast shutter speed around 1/2000, which doesn’t let much light into the sensor. To compensate for the quick shutter speed, use an ISO of 200.