Adjusts the brightness of the screen.
The screen brightness is usually the most important factor influencing the battery life of your device, and using a low screen brightness has the potential to lengthen your battery life considerably, at the cost of being able to see the screen well during bright conditions such as outdoors during the day.
Most devices provide an "auto brightness" setting as part of the brightness setting, which allows the device to detect how bright its surroundings are (ambient brightness) and adjust the brightness according to that. In theory, this should result in a good balance between saving battery usage, and being able to see the screen well in bright conditions. The behavior and sensitivity of the auto brightness setting can vary between devices.
When in "auto brightness" mode, some devices allow further manual override to the brightness setting. This is achieved by turning "auto brightness" on and then further adjusting the brightness by dragging the control to the left or right. In devices where this isn't supported, the brightness control bar will simply disappear or become disabled while auto brightness is turned on.