“Auto-brightness” is a setting that is intended to save battery by automatically brightening or dimming your iPhone’s screen based on the ambient light where you are.
When the “auto-brightness” setting is turned on, if you are in a dark place then auto-brightness will automatically dim your iPhone’s screen (as you generally would not need your iPhone to be really bright in order to be able to read your iPhone in a dark place), and if you are in a bright place (like outside in sunlight) then auto-brightness will automatically brighten your iPhone’s screen so you are able to see the screen.
The “auto-brightness” setting is automatically turned on in iOS 11. However, if you are like me and prefer to have your screen brightness held constant all of the time, then “auto-brightness” can get pretty annoying. Luckily there is a way to turn it off, and here is how.