CFI, that's ok, I don't mind.
I did use the phrase "partial vacuum", but "lower pressure than ambient" will work also. The main idea to understand is that the "pumping action" of the engine will lower the internal pressure of the intake manifold below that of the surrounding atmosphere, if the throttle is partially closed. The manifold pressure gauge will in turn read this as a absolute barometer value below the ambient pressure.
If a manifold pressure gauge ever read "0" then the engine would die, as it cannot run without some "atmospheric pressure" inside the intake manifold. You cannot burn fuel without the oxygen from the air.