Sometimes, someone who has been waiting for a visa or another immigration status will want or need to travel abroad, but they may not have a passport or other valid status that will let them back in the United States upon return. Alternatively, sometimes someone in your family may need to be let into the U.S. quickly, especially in cases such as family emergencies. In both of these situations, the remedy is called parole - the latter is humanitarian parole, while the former is called advance parole. Both can be difficult - but not impossible - to get.
Advance Parole (AP) is exclusively for those who are not in legal immigration status - for example, those who have been granted asylum or refugee status, those who are in the middle of adjusting status, and those who have been granted Temporary Protected Status (TPS). In some cases, it may be able to be granted for those who still have an asylum claim pending before the courts. AP allows someone who belongs to any of these groups, or a few select others, to be able to travel abroad and return to the country to continue waiting for their official immigration status to vest.
AP is necessary because, without it, anyone who lacks valid status essentially cannot leave the United States for any reason, no matter how serious. They would have no documents to show upon arriving back in the country and would be turned around in all but the most unusual cases. This is a problem not only because they may have a claim to legal status, but also because in some cases, spending too long out of the country will make U.S. Customs and Immigration Services (USCIS) determine that you have abandoned your application for status entirely. USCIS states explicitly that before leaving the U.S., anyone without legal status must have already been approved for advance parole....