Hi, in a simpified way, a mass airflow sensor measures the amount (mass) of air flowing into the engine at any given time. It has an internal electronic circuit which measures the current required to keep a reference temperature of a resistor array. since flowing air will cool down the array, the more current required to reheat it the more air mass is flowing. The current is translated into an electrical signal used by the electronic control unit to deliver the required amount of fuel by controlling the time that the injectors will remain open. This action happens in milliseconds and repeat itself as long as the engine is running.