Plain English Meaning
The highest pressure the engine maker says you are allowed to have in the intake system for a given phase of flight. Push past it and you risk damaging the engine.
Full Definition
The highest manifold pressure the engine manufacturer permits for a given operating condition, such as takeoff, climb, or cruise. Exceeding this limit risks detonation, excessive cylinder pressures, and engine damage, and is especially relevant in supercharged or turbocharged engines where the induction system can produce pressures well above what an unboosted engine could ever generate.
Context Anchor
Seen when setting power in aircraft with superchargers or turbosuperchargers, especially during takeoff, climb, and - operation.