ISO

Definition: ISO is the sensitivity of the camera to the amount of available light in the environment.

When you change ISO, the digital sensor becomes less sensitive or more sensitive to the light around it depending on what you change it to. With less light sensitivity, you get a less grainy and more clear photo, and with more light sensitivity, you get a more grainy and less clear photo.

In order to balance the light meter doing ISO, you need to change both aperture and shutter speed according to the ISO being used to take the photo.