This allows you to automatically turn on low power mode while your device is locked. Once you unlock it turns off low power mode. You can also set a battery level to automatically turn on low power mode.
Low power will also stay on while charging when the device is locked to facilitate faster charging. I have tested this on my iPhone X on 11.1.2 and need results from other devices and iOS versions. It should work for 9.3+ but I do not have anything to test on.