ISO is the third element of the exposure triangle. It refers to the sensitivity of the camera's sensor to light. The higher the ISO, the more sensitive the sensor is to light, and the brighter the image. However, increasing the ISO also increases the amount of digital noise in the image, which can reduce the quality of the image. Low ISOs are great for capturing images in bright light, while high ISOs are perfect for low-light environments. Understanding the relationship between these three elements is crucial for creating a balanced exposure. For instance, if you want to create a shallow depth of field, you will need to use a wider aperture, which means that you will need to compensate for the increased light by using a faster shutter speed or a lower ISO.