In Digital Photography ISO measures the sensitivity of the image sensor, the lower the number the less sensitive your camera is to light and the finer the grain. https://digital-photography-school.com/iso-settings/
These photos are taken in a light area it goes in order: 1600, 800, 400, 200, 100.
These are photos are taken in a really dark light and once again goes in order: 1600, 800, 400, 200, 100.