Plain English Meaning
The GPS receiver stops getting a good enough signal from the satellites to tell you where you are.
Full Definition
In GPS operation, the interruption or weakening of satellite signals reaching the receiver to the point that the receiver can no longer compute a reliable position. Signal loss can be caused by terrain or building obstructions, aircraft attitude that blocks the antenna's view of satellites, interference, jamming, or insufficient satellite geometry.
Context Anchor
Seen in GPS navigation discussions, cockpit GPS messages, and situations where terrain, aircraft structure, antenna problems, or interference can keep satellite signals from reaching the GPS unit.