"Automatic brightness" refers to a feature in devices such as phones or TVs that adjusts the brightness level of the screen automatically based on the surrounding light conditions.
The update also promises faster cover loading along with automatic brightness control. (goodereader.com)
It's also worth noting how useful automatic brightness settings on a smartwatch are, which are enabled by the ambient light sensor embedded in that flat spot. (theverge.com)
This can also interfere with automatic brightness because when the phone is tucked away the ambient light sensor is hidden. (factschronicle.com)