ISO refers to a camera sensor's sensitivity to light, with higher ISO numbers indicating greater sensitivity. The document recommends setting a camera to ISO 1600 or higher when taking photos indoors, where it will be darker, to ensure proper exposure without needing to use a flash. It also notes that higher ISO settings can introduce more noise or grain into the resulting photos. The photographer is advised to set their camera to ISO 1600, use shutter priority mode with a shutter speed of at least 1/60th, brace the camera against a wall if needed, and avoid using a flash.