*edit*Curses! Kurfurst beat me to it! I'll leave the comic in anyway...*/edit*
Boost and manifold pressure are the same thing; a measure of the pressure in the intake manifold (ie the ducting between the supercharger and the cylinders).
The British convention was to measure this pressure in psi gauge and call it boost; zero boost means that the pressure in the intake manifold is nominally atmospheric (14.7 psi absolute), +12 means atmospheric pressure + 12 psi (i.e. 24.7 psi absolute) and so on. So this system works rather like the pressure gauge on a bicycle pump (except that the scale goes all the way down to -14.7 psi, which would be a perfect vacuum).
Almost everybody else decided to measure manifold pressure in absolute units, such that zero would be a perfect vacuum.
The Americans use inches of mercury; the Germans use ata.
The ata is a rather annoying little unit, because it's not quite what you'd expect.
A standard atmosphere is 101325 Pa
A bar is 100000 Pa
An ata is 98066.5 Pa (ie 1 kgf/cm^2).
So when your engine is switched off on a standard day at sea level you'd expect to see a manifold pressure of:
101325 Pa = 0 psi boost = 29.92"Hg = 1.0332 ata.
So you can see how lightly boosted the German engines were: 1.42 ata is only 20.2 psi absolute or about +5.5 psi boost.
Why do we care?
Too much pressure in the intake manifold means too much pressure in the cylinders, which will cause problems for the engine.
In particular, detonation. High pressure and long combustion chamber residence times increase the probability of detonation, so you can only get away with so much manifold pressure at any given rpm; you can get away with higher boost at higher rpm, but of course there's an rpm limit too...
Also, with a constant speed prop the rpm we get is the rpm we asked the CSU to give us, and this therefore doesn't tell us anything about how much power the engine is producing.
Since the power production is basically a function of the engine's air consumption, it will vary roughly in proportion to engine rpm * air density in the intake manifold.
Since we can't measure the density of the air in the intake manifold directly, pressure is a pretty good substitute because we know that
P*V = roh*R*T
All this stuff about engines needing vacuum to run just isn't true. The engine wants as much air as possible. It only creates a depression in the intake manifold when we throttle it. It can't get as much air as it wants to, and so it sucks down the pressure in the intake manifold.
But if you open the throttle of a naturally aspirated engine then the pressure in the intake manifold will rise to (almost) ambient pressure.
If the engine is supercharged, the throttle is almost always upstream of the supercharger.
An early Merlin has a supercharger pressure ratio of about 2.7 at 3000 rpm. So if the engine ran unthrottled at sea level then the manifold pressure would be about 2.7*14.7 = 39.7 psi, which is +25 psi boost. This would cause rapid engine failure due to detonation, and even if you put 150 grade fuel into it, an early Merlin just wouldn't be physically strong enough to stand up to the cylinder pressures produced for more than about 15 minutes or so (though for that glorious 15 minutes it would put out over 2000 bhp).