The mass airflow (MAF) sensor includes a piece of very fine wire with an electronic device that measures the amount of air being sucked into the engine and relays that information to the electronic control unit. The wire is heated when power passes though it.
If dirt or oil comes in contact with the wire, the ability to measure the passing air is diminished.
In this video, Andrew Markel explores the possible sources of contamination of the MAF sensor.
This video is sponsored by NGK and NTK.