I have a 1998 Beetle.
What is the large flat plastic piece (it's a large, thin square piece) on the underside of the chassis - directly under the engine?

The reason I want to know - it fell off today.
Do I need to be concerned?

It keeps the airflow under the car from getting turbulent. It also aids in keeping a smooth flow of air through the radiator & over the engine, but it's loss will probably not have a noticeable effect.