A mass flow sensor is a sensor used to determine the mass flow rate of air entering a fuel-injected internal combustion engine. The air mass information is necessary for the engine control unit to balance and deliver the correct fuel mass to the engine. Air changes its density with temperature and pressure.
The computer in your car measures electrical resistance through the Mass Air Flow (MAF) sensor as the air flowing through the sensor cools its components down. With a firm understanding of how much air is flowing into your engine, the computer can then direct fuel injectors to add more or less gasoline to the air/fuel mixture.
It is installed between the air filter and the intake manifold of the engine. There are few types of air flow sensors, however, modern cars use a hot-wire type. Problems with mass air flow sensors are common in many cars.