It is an Air Flow Meter.  It is attached to your air box between it and a
rubber boot that goes to the throttle plate, and basically tells your fuel
injection computer how much air is coming into the engine so it can give you
a proper mixture of fuel and air in the cylinders for an efficent burn.

They have a tendancy to wear out over time and can be a source of the
infamous "bucking" that many Vanagon owners have experienced at one time or
another.

Hope this helps.
Ken Wilford
Van-Again
John 3:16