As stated in the family-based page, there are certain bars to becoming a legal permanent resident. One of those bars is unlawful presence in the US. An undocumented immigrant in the US may be eligible for a provisional waiver if the immigrant has a US citizen or legal permanent resident spouse or parent. In order to qualify for the waiver, you must be able to demonstrate that refusal of the legal permanent residence would cause extreme hardship to the spouse or parent. The provisional unlawful presence waiver process allows individuals, who only need a waiver of inadmissibility for unlawful presence, to apply for a waiver in the United States before they depart for their immigrant visa interview at a U.S. embassy or consulate abroad.
Under the previous process, immigrants had to apply for the waiver while in their home country and wait a year or more abroad. While under the current law immigrants still need to attend an interview in their home countries, the time spent abroad is general only a few weeks.