The Manifold Absolute Pressure (MAP) sensor measures the pressure inside the intake manifold. Unlike MAF sensors that measure air weight directly, the MAP sensor measures the vacuum created by the pistons. When the engine is idling, manifold pressure is low (high vacuum). When you step on the gas, the throttle plate opens, and pressure rises to atmospheric level. The computer uses this pressure data along with engine RPM to calculate engine load and calculate fuel delivery.
On turbocharged engines, the MAP sensor also measures boost pressure. If the sensor fails, the computer cannot calculate fuel needs correctly. The engine will run rough, blow black smoke from the exhaust, idle erratically, or stall when you accelerate.