The Airplane mode switch allows you to temporarily disable all wireless communication on your device including Wi-Fi, mobile networks and Bluetooth.
This feature is sometimes called Flight mode instead.
Wireless communication will remain disabled until you turn off airplane mode, and then they will each return to their previous settings.
Airplane mode is named because it allows your device to be used on an airplane on which wireless communicating devices are not allowed to be used.
On modern devices it is usually possible to enable Wi-Fi while in airplane mode, using the regular Wi-Fi switch in the settings or in the notification shade.
Wi-Fi will be disabled at first each time you enable airplane mode, and you will need to then turn Wi-Fi should you wish to use it while in airplane mode.
This may be useful if you are travelling on an airplane which specifically allows Wi-Fi to be used but not any other form of wireless communication.