Complex sentence: There are six picnic tables near the back of the room that are only used when the rest of the restaurant is full and there are guests who are willing to forego the experience of sitting in the cars.
Split into simpler sentences: There are six picnic tables near the back of the room that are used as overflow seating. These tables are only used when the rest of the restaurant is full and there are guests who are willing to forego the experience of sitting in the cars.