In order to obtain U.S. lawful permanent residence (a green card), many people who have spent more than 180 days in the U.S. illegally must apply for a waiver of that unlawful presence in order to avoid a time bar of three or ten years. For those living in the U.S. already, however, applying for the waiver at an overseas consulate, while attending their immigrant visa interview, carries the risk that it will be denied and they will be stuck overseas for the time-barred period. That's why the provisional waiver was created--to allow applications to be made before departing the United States. It's a complicated subject, however. Learn more here.