It's one of those codes that drives you or your mechanic crazy to find. Common to most Volkswagen cars and many others, the engine code indicates that the air flow passing through the MAF sensor is out of range for some reason or intermittently out of range, which may make your car seem okay and not in limp mode (unable to speed past 2500 rpm). Of course, around town driving, if your VW is in limp mode, you may not even know it.
The OBD II code is aggravating to locate. The code is rather specific to air intake problems because the MAF sensor is very sensitive. Options to locate it are time consuming and can be costly to fix a simple problem. Yet, the engine light will remain on. Of course, there may be other codes also. Anything out of the spec for your car will trigger the engine light. If it flashes, it is very serious.
For the P0101 code, make sure the air intake areas are not blocked at entry points or within the hoses, the MAF many need cleaning, the air filter may have allowed tiny dirt particles through to the MAF, the connector harness may need cleaning or better contact, the wiring to the connector should be tested to make sure the connector is getting proper voltage etc.according spec. Make sure there are no vacuum leaks in the upper hose that connects to the MAF and the smaller flex hose directly under it. The smallest leak in either will cause the code. Finding the small vacuum leaks are very difficult. If you suspect it, the readings on an engine analyzer will reflect it. If you MAF is bad, you should replace it. Of course, a more uncommon problem is that your car engine computer has malfunctioned.