Airplane or flight mode disables all those features of the device that require wireless communication. These include Wi-Fi, Bluetooth, incoming and outgoing calls and also SMS. A typical modern cell phone communicates with multiple cell towers and tries to maintain this connection all the time. If the towers are far away, the device must amplify the signal it transmits in order to be able to communicate with the towers. This type of signal emission can interfere with the aircraft sensors and can cause problems for the aircraft's sensitive navigation equipment. Fortunately, today's modern airplanes are equipped with enough advanced equipment that help the plane to not crash, just because some people have forgotten to turn on flight mode on their devices.