Airplane mode seems simple enough: Just toggle this feature when you're on an airplane or other offline situation, shutting down all of the device's radios and saving battery life. But in Windows Phone, Airplane mode is a bit more nuanced, and in Windows Phone 8.1, in particular, it's far more accessible too.
First, the new bits: Airplane mode is now available from Action Center, so you can quickly and easily toggle this mode as needed. Just tap the Airplane Mode quick setting tile. When it's lit up, Airplane mode is enabled, and all of your handset's radios have been turned off. You'll also see an Airplane Mode symbol in the status bar at the top of the screen.
You can also access Airplane mode the old fashioned way: Navigate to Settings, System, Airplane Mode.
Using Airplane mode toggles Wi-Fi, Bluetooth and your cellular connection, of course, but it also impacts NFC (Near Field Communication), which was sometimes called Tap + Send in Windows Phone 8.0. So when you turn on Airplane mode, all of those radios are turned off. When you turn Airplane mode off again, only the radios that were previously on will be turned on. So if you had Bluetooth off previously, Bluetooth won't be enabled when you turn off Airplane mode.
The interesting wrinkle to Airplane mode on Windows Phone is that once this mode is enabled you can separately toggle Wi-Fi, Bluetooth, and NFC. So you might enable Airplane mode, turning off all of your handset's radios, but then enable Wi-Fi. In this case, only Wi-Fi would be enabled, and your cellular connection, Bluetooth and (if available) NFC would remain off.
This can be useful if you're on an airplane that has Wi-Fi: You can turn on Airplane mode but then separately re-enable Wi-Fi. This way, you will be able to get online but still take advantage of the battery life savings afforded by having the other radios off and, in the case of cellular in particular, constantly trying to connect to a signal.
To bring this discussion back to Windows Phone 8.1 specifically, you can of course use the Wi-Fi quick setting tile to toggle Wi-Fi back on while Airplane mode is on. Just tap it a second time to visit Wi-Fi settings and connect to an available Wi-Fi network.