by Fran T
Keeps the display active while you're reading, allowing it to turn off when the phone is locked, in a call, lying faceup, in power save mode, or when in close proximity with something (as when you're in a Hangouts call or the phone is in a pocket or purse, or lying facedown). And unlike certain vendor-supplied camera-based features, it works in the dark!
The phone's current display timeout system setting is used to determine the minimum amount of time the screen will stay on. With this flow, you can improve your phone's battery life by setting your display timeout to the minimum value and it will stay on only as long as you need it.
Also see the Android Settings --> Display --> Screen timeout and Motion sensor calibration settings.