Many people who have flown have heard the cryptic announcement “flight attendants, please prepare for cross-check” over the public announcement system. This is a procedure that is performed by flight attendants before a plane pushes away from the gate, and again when the plane lands before the doors are opened so that passengers can disembark. The flight attendants check to make sure that the doors have been “armed” with emergency escape slides at this time.
When the doors of an aircraft are armed, they are attached to inflatable slides that will pop out and inflate automatically when the doors are opened, allowing people to quickly escape from the aircraft. After a plane is first loaded with passengers, the flight attendants close and arm all of the doors when instructed to do so by the pilot as part of the routine preparations for take off. During the cross-check, the flight attendants double-check that all the doors are armed, reporting to the head flight attendant.
In many areas, a plane cannot push back from a gate or stairway until the doors have been armed, in case there is an emergency on the tarmac that requires evacuation. The emergency slides can also be used as flotation devices, in the event that a plane crashes in water and passengers survive the crash. The cross-check is one among a series of procedures that are designed to ensure that a plane is safe for flight.
Once a plane lands, the doors need to be disarmed before the passengers can disembark because, otherwise, the slides would inflate when the doors are opened. Once the plane has been brought to a full stop, the pilot asks the flight crew to disarm the doors and check them to ensure that they have in fact been disarmed, so that they can be opened safely, allowing passengers to deplane and continue on their way.
Incidentally, while the safety announcements on planes may be dull, it's a good idea for passengers to listen to them, because the layout of each airline's planes is slightly different. Even if a person has ridden on a particular type of aircraft before, he may not be familiar with the safety procedures for the plane he is actually on. By paying attention to the safety lecture, passengers can ensure that they will know what to do during an emergency.