Airplane mode disables the antenna(s) on your phone and stops transmissions related to your cell signal and WiFi access (if your phone is capable of that), allowing you to safely and legally use it on an airplane during the time when electronic devices can be used in-flight.
You won't be able to make calls, or text, or transmit any data, but you CAN use the camera, listen to music, play any games (provided they are not data connected).
You will need to go into Settings > Wireless & networks and then uncheck the box next to Airplane Mode to disable it. It will not automatically reset when you restart the phone.
