Plain English Meaning
The gauge that shows how hard the engine is being pushed is reading lower than it should for the throttle setting you have selected. In a turbocharged airplane, this is often the first sign that the turbocharger has stopped working properly.
Full Definition
An indication on the manifold pressure gauge showing intake manifold pressure below the value expected for a given throttle setting, altitude, and power demand. In a turbocharged airplane, low manifold pressure during climb or cruise is a primary symptom of turbocharger failure or wastegate malfunction, indicating that the system is no longer compressing induction air to the commanded .