You can see the lever that the flight attendents need to flip each time they get to a gate and then leave. Right by the emergency exit door on the small lever it has two positions that say something like manual or automatic. This just means that the inflatable slide that would come out in an emergency won't pop up if the aircraft is at the gate.
The cross check (as FlightSimFreak explained) is just having the F/A across from you double check the position. It has happened before that a F/A has forgotten to disarm the emergency slide at the gate and it has inflated. As far as I know, however, this has never happened at the boarding door (that would be real bad).
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