I’ve read a lot of potentially confusing terminology on this thread. I’ve never liked the term “vacuum”, especially because the aircraft doesn’t have a vacuum gauge.
During my days of teaching piston engine theory to basic students, some of whom had no prior knowledge, yet were learning to fly from scratch on aircraft with a CS propellor, I found it useful to point out that the engine is actually a petrol powered air pump and the MAP gauge simply measures the state of affairs inside the inlet manifold.
The engine is able to pump out air from the inlet manifold, this is indicated by a reduction in MAP. The atmosphere is naturally trying to refill the manifold, indicated by an increase in MAP. The throttle plate is a valve which restricts the rate at which the atmosphere can refill the manifold.
The MAP shows the result of a balance between how hard the engine can suck (RPM related) and how quickly the prevailing atmosphere can find its way past the air intake assembly (density related) and the position of the throttle plate (pilot controlled).