I find it useful to remember that a piston engine is an air pump. The engine tries to suck the air out of the inlet manifold and the atmosphere does its best to keep it filled. At higher rpm and/or lower throttle openings, the engine will win (giving a relatively low MAP reading) and at low rpm/larger throttle openings the atmosphere will win (giving a relatively high MAP reading).
A normally aspirated engine can, even at full throttle, only ever have at best, ambient atmospheric pressure in the manifold. A forced induction engine can have more than atmospheric pressure, mainly depending on throttle position, although rpm does have an effect. As already stated, with increasing altitude/reducing air density, eventually the manifold air pressure would reduce to be less than normal sea level pressure. That's where the two stage (or alternatively, two speed) supercharger comes in handy. The later Merlin engines had one supercharger blowing into the second one - they ran in series.
I have a supercharged car. It has a boost gauge (which naturally reads close to "zero" with the engine switched off). When driving long distances on road I normally try to keep the boost gauge at a negative figure by using a very small throttle opening because that results in better economy and stresses the engine less. As soon as I open the throttle further so that the boost gauge reads more than the "zero" position, I'm aware that I'm asking the engine to do more than it could if the supercharger wasn't fitted.