The battery saver mode allows you to significantly extend the battery life of your device. Once this mode is enable all unnecessary services will be disable ( GPS, background services, auto-update...etc).
To enable the Battery Saver Mode of your Nokia Lumia smartphone:
Tap on the Start key > Settings > Battery Saver
"Always turn on Battery Saver mode when battery is low" - To automatically switch to battery saver mode once the phone's battery has gone down to a certain level.
"Turn on Battery Saver mode now, until next charge" - Switch immediately to Battery saver mode.