MAF is a Mass Airflow Sensor, it is an electrical component thats connected to the intake air duct just after the aircleaner.
you should see a wire harness going to the air duct this is the MAF.
It measures the density and temperature of the incoming air and then sends a signal to the computer to add the correct amount of fuel to the injection system

 

The MAF is the Mass Air Flow sensor; it's that thing between the air intake filter box and the hose to the intake manifold. Its job is to tell the ECU how much air (specifically, how much oxygen) is making it to the cylinders.

Before replacing either it *or* the O2 sensors, I think I'd try to pin down which is the problem; none of them are exactly cheap parts, generally, and they're all the sort of part you wouldn't generally want to install used.