ISO stands for International Standard Organisation.

ISO is a number that indicates how sensitive your camera is to the available light.  The ISO ratings are auto,100, 200, 400, 800, 1600, 3200, 6400. The ISO setting is directly set up with your shutter speed and aperture setting, to make the photos properly exposed. When the ISO rate is higher less light would be needed to obtain enough exposure. Over all changing the ISO settings on the camera increases or decreases the sensitivity and light gathering ability 

ISO 400

ISO 800

ISO 1600

ISO 200

ISO 100

ISO Auto