Manifold Absolute Pressure (MAP)/ Manifold Pressure sensors are used to determine the pressure of the air inside the inlet manifold, just before it enters the engine inlet ports. This is generally measured between the Throttle Body and the intake ports on a naturally aspirated engine, and between the turbocharger compressor outlet (after the Intercooler) on turbocharged or supercharged engines.
This is an important metric for the ECU and is used in combination with the Intake Air Temperature (IAT) to determine the density of the incoming air. On naturally aspirated engines, the throttle body effectively controls the manifold pressure. Ambient air pressure changes a small amount day-to-day, and by larger amounts at different altitudes, and on boosted engines even more significantly due to the engine operating point. Changes in the pressure of the intake air are likely to cause changes in its density, which effectively changes the amount of oxygen entering the engine. For correct combustion, we wish to have a particular mix of air (oxygen) and fuel.
As an example, if we drive from sea level where air pressure is typically around 1,000 milliBars to the peak of Everest (somehow) the pressure would fall to around 300 milliBars1, less than 1/3 as much which, according to the Ideal Gas Law also reduces the density by 1/3 and hence your engine will receive 1/3 as much oxygen and will require 1/3 as much fuel to keep a stoichiometric ratio. This is the reason that the later piston engine fighter planes such as the Supermarine Spitfire were fitted with two-speed superchargers2