ISO, or International Organization for Standardization, refers to the sensitivity of the camera sensor to light. A higher ISO setting will make the sensor more sensitive to light, resulting in a brighter image. However, increasing the ISO setting also introduces noise or graininess in the image. This is because amplifying the signal to make the image brighter also amplifies any electronic noise present in the sensor.
On the other hand, a lower ISO setting makes the sensor less sensitive to light, resulting in a darker image. In order to achieve a brighter image with a lower ISO setting, one must compensate by using longer exposure times or wider apertures. This allows more light to reach the sensor without introducing as much noise.
In summary, ISO affects the brightness of an image by determining the sensitivity of the camera sensor to light. A higher ISO setting will result in a brighter image but also introduce more noise, while a lower ISO setting will result in a darker image that may require adjustments to exposure time or aperture to achieve the desired brightness.