It uses two heated-wire sensing elements to measure the amount of air entering the engine.
Air flowing over the two heated wires causes them to cool. As they cool their resistance changes. That causes a variation in an analog voltage signal to the PCM (power control module) proportional to changes in the intake air mass.
The PCM uses this analog signal to calculate the fuel injector pulse width needed to provide the desired air/fuel ratio. On cars with an automatic transmission, this input is also used to determine transmission electronic pressure control, shift and torque converter clutch scheduling.
If you would like more information about how a MAF works on a Mustang, you can find an excellent tutorial here
