ISO is one of the three legs of the exposure triangle used to make sense of what goes into determining an exposure. The other two legs are aperture and shutter speed. ISO is the sensitivity to light as pertains to either film or a digital sensor. A lower ISO number means less sensitivity and a higher ISO number means more sensitivity. Film has a single ISO rating, meaning that if you put a roll of ISO 400 film in a camera, you will be shooting at ISO 400 for the entire roll. Digital sensors can be set to various ISO speeds depending on camera model. For example, a Canon 5D Mark IV’s native (not including expanded ISO) ISO range is ISO 100 - 32,000. Each doubling or reducing by half of an ISO number equals an addition or reduction in sensitivity amounting to one stop of light.