Plain English Meaning
It is the GPS's self-check that confirms it has enough good satellite signals to be trusted for navigation in the busier, more precise environment near the airport.
Full Definition
A Receiver Autonomous Integrity Monitoring (RAIM) check performed by a GPS receiver for the terminal phase of flight, which begins 30 nautical miles from the destination airport. It verifies that enough satellite signals are available, with sufficient geometry, to provide the tighter accuracy and integrity required as the aircraft approaches the terminal area.