The third and final part of the exposure triangle is called the ISO.
The ISO controls how sensitive your camera’s sensor is to light. If photographing in a dark scene, you would need to increase your ISO because more light is needed to properly expose your subject. If however you are photographing in a bright environment, you would decrease your ISO. BE CAREFUL THOUGH!!!! High ISO settings will create extra grain in your images making them look fuzzy instead of sharp.
ISO increases by double the amount before it and the larger the number, the more sensitive the camera will be to light (See below for example).
Check out this great diagram from digitalcameraworld.com to learn more.