ISO is a camera setting that adjusts the sensitivity of the cameras image sensor to light, and it is one of the three pillars of photography, along with shutter speed and aperture. Increasing the ISO number makes photos brighter, while decreasing it makes them darker. ISO is measured in numbers, and common ISO values include 100, 200, 400, 800, 1600, 3200, and 6400. The higher the ISO number, the more sensitive the camera is to light, which can be useful in darker environments or when you need more flexibility with your aperture and shutter speed settings. However, increasing the ISO too much can result in graininess or noise in the photo. In summary, ISO is an important camera setting that affects the brightness and quality of your photos, and it is one of the key elements of the exposure triangle in photography.