ISO controls your digital camera's light sensitivity, with lower numbers (e.g., 100) being less sensitive to light and ideal for bright conditions, producing cleaner, less noisy images. Higher ISO numbers (e.g., 1600+) increase sensor sensitivity, making the image brighter and allowing for faster shutter speeds in low-light situations, but also introducing digital noise or "grain" into the photo. The goal is to keep the ISO as low as possible to maintain the best image quality while achieving proper exposure.