The MAF tells the ECM how much air is flowing through the engine. That information is used by the ECM to help determine how much fuel should be injected for the proper fuel/air ratio. If the MAF fails, which is pretty common, the result can be hard starting, a rough idle, or bad overall performance. When you unplug the MAF the ECM will use data stored in memory to replace the information the MAF normally provides. Sometimes the engine will run better with the MAF unplugged giving you an indication that the MAF is bad.