The MAP sensor (Manifold Air Pressure Sensor) is a main input to the ECM or engine computer. The MAP
sensor measures the difference in air pressure between the intake
manifold and atmospheric pressure. The ECM infers engine load from the
MAP sensor reading.The MAP sensor converts engine vacuum and boost pressure into a varying output voltage, between 0-5 volts.
Manifold: The primary function of the intake manifold is to evenly
distribute the combustion mixture (or just air in a direct injection
engine) to each intake port in the cylinder head(s). Even distribution
is important to optimize the efficiency and performance of the engine. (source: wikipedia)