Air Flow Meters
http://www.auto-sensors.co.uk/air-flow-meters-72-c.asp Air mass meters are also commonly referred to as air flow meters. They measure the mass of the air flowing into the engine. The air mass information is necessary to calculate and deliver the right amount of fuel to the engine. Usually situated directly after the airbox so that filtered air is measured.
Simply put, they are a plastic tube that air passes through. There will be sensors inside that are in the air stream. There will be a plug with electrical wires attached to it. These send the air mass meter information to the cars computer (ECU).