Airplane mode disables all wireless transmissions: mobile data, calls, SMS, Wi-Fi, and Bluetooth. Required by aviation law during takeoff and landing to prevent potential interference with aircraft instruments.
After enabling airplane mode, you can individually re-enable Wi-Fi and Bluetooth for in-flight Wi-Fi or wireless earphones.
Beyond planes, it's useful for focusing on study or work (blocks all calls and notifications), and for saving battery (wireless transmission is a major power drain). Charging is faster with airplane mode on.
Important: emergency calls (110/119) cannot be made in airplane mode. Disable it before calling for help.