Plain English Meaning
The air-driven system that keeps certain flight instruments spinning has stopped working, so those instruments will slowly give wrong readings.
Full Definition
A loss of the engine-driven vacuum or pressure source that powers gyroscopic flight instruments, typically the attitude indicator and heading indicator. When the pneumatic system fails, the affected gyros gradually spin down and their indications become unreliable, often without an obvious warning to the pilot.
Context Anchor
Seen in instrument flying when a pilot must recognize that air-driven instruments are no longer trustworthy and use other available instruments to keep of the .