ISO represents the sensitivity of the light sensor within the camera. Low ISO means the camera’s sensor is less sensitive to light, and high ISO more so. More sensitivity is not always good. Higher ISO in relation to shutter speed or aperture can result in pictures filled with digital noise, which looks grainy. If ISO is too low, a photo will be underexposed.
ISO used to refer to film, not cameras. Different films were more or less sensitive to light, and ISO was a way to quantify that sensitivity. Film still comes with your choice of ISO, but now ISO is associated more with adjustments you can make in digital cameras.
“ISO comes into play when you want to capture action,” says photographer Heather Barnes. “Let’s say you have a small aperture, maybe like an f/16, but you’re still not getting enough light in your scene. That’s when you want to bump up ISO.”