ISO refers to how sensitive the camera’s sensor is to light. The higher the ISO sensitivity number, the less light is required to make an exposure. Use ISO, coupled with aperture and shutter speed, to achieve the “correct” exposure.
Auto ISO: When this menu function is chosen, the camera will automatically, almost magically, select the necessary ISO if optimal exposure can not be achieved at the ISO value you’ve dialed in. Turn this feature ON, then just shoot away. You won’t have to worry about lighting or shutter speeds – just concentrate on composition.