What does airplane mode actually do?

Airplane mode disables all of your phone's radios, the components that transmit and receive wireless signals: cellular (calls, texts, mobile data), Wi-Fi, Bluetooth, and GPS transmission. With them off, the phone stops communicating with cell towers and networks entirely, which is why it was created, to satisfy aviation rules meant to prevent any chance of phones interfering with aircraft systems, a caution that persists even as the actual interference risk is now considered minimal. The name stuck even though its uses have expanded well beyond flying.

Useful things to know: on most phones you can turn airplane mode on and then re-enable Wi-Fi and Bluetooth individually, which is exactly what you do to use in-flight Wi-Fi or wireless headphones while keeping cellular off, a common point of confusion. Beyond flights, airplane mode is genuinely handy: it charges your phone noticeably faster (the radios are among the biggest power draws, and disabling them lets more energy go to the battery), it stops all notifications and calls for instant do-not-disturb, it saves battery in areas with weak signal where the phone would otherwise exhaust itself hunting for a tower, and toggling it off and on is a quick fix that forces the phone to reconnect fresh to the network, often resolving a stuck or dropped connection. It also stops using roaming data abroad while still letting you connect to hotel Wi-Fi.