Airplane Mode is a feature built into your iOS devices that makes your devices airplane friendly by disabling all the wireless radios that could be a nuisance to on-board plane equipment, but it can also be used to increase your battery life.
Since the wireless radios are put to rest when your device's Airplane Mode option is enabled, you'll burn through less battery on a single charge with the feature turned on, and this is basically the idea behind a new jailbreak tweak dubbed Airplane Enhancer+.