An immigration hold can mean different things. Usually, it refers to an alien who is detained by state or federal authorities. If your boyfriend is in custody of a state or federal detention facility, it means that as soon as he is released from a state or federal custody, ICE (Immigration and Customs Enforcement) will take over the custody and continue to hold your boyfriend in their custody.
You can find out about this by calling the jail/prison; or you can call local ICE office. You should have his Alien number ready.
An immigration hold is placed by ICE (Immigration Customs Enforcement) when someone they believe is an illegal alien is in state custody (or federal). Most jails (I say most because smaller counties may not have an advanced webpage) or sheriff's offices have websites. Some even let you do an inmate search and it will tell you what they are charged with and if there are any holds, i.e. out of county hold, out of state hold, or immigration hold.
Once the immigration hold (or detainer) has been placed on an inmate, ICE only has 48 hours (excluding weekends & holidays) to pick the inmate up ONCE THE COUNTY'S INTEREST IN THE INMATE IS FULFILLED. Now, most people make the mistake of thinking that is only when the inmate's case is over and he/she has finished serving their sentence. That is also true, however, if the inmate posts a bond: the county's interest is fulfilled and the 48 hour clock starts to tick!
If the 48 hours pass and ICE has not picked the inmate up, you need to hire an attorney to get the inmate out. Oftentimes this can be done by placing a phone call to the legal counsel of the sheriff's office; if that doesn't work, your attorney will need to file a Writ of Habeas Corpus with the court.
You can always call the jail to find out if there's a hold.
An immigration hold basically refers to a document filed by the U.S. Immigration and Customs Enforcement or I.C.E. (the agency responsible for immigration detention and removal) which asks the law enforcement agency holding someone to release that person upon completion of their criminal sentence or detention to I.C.E.
At that point, the agency usually conducts an interview to determine whether or not that individual is subject to being removed from the U.S. They also decide other issues like: 1) Does the person have a right to an immigration court hearing, or can they be removed from the U.S. without a hearing? 2) Can the individual be criminally prosecuted for illegally reentering the U.S. after removal? 3) Is the individual eligible to be released on bond or not? and 4) Various other issues.
If someone knows they have an immigration hold, they should seek the assistance of an immigration attorney familiar with these issues as soon as possible. Acting quickly in these cases often can make a huge difference.