The fries that are requested at a restaurant prior to drinks, appetizers, and main course orders are placed. Prefries are usually ordered when first sitting down at the table from restaurants known for having "bottomless fries" deals, and are intended to quell the hunger of all attending the meal.
Waiter: Right this way guys.
Cary: Could we get 3 baskets of prefries please
Waiter: Sure, no problem.
John: Could one of those baskets be garlic prefries and a side of tartar sauce?
Waiter: We can do that for ya.
All (in unison): Thanks!