The manifold refers to an inlet in the engine that receives air and fuel from the carburetor and then transfers it to the cylinders. The manifold pressure gauge measures the air pressure going through the manifold inlet. Just like other pressure readings, this is measured using inches of mercury in the States, and hectopascals elsewhere. In normally aspirated engines, the pressure is lowest when the airplane is at idle and highest when at full throttle and maximum RPM. Remember that air density