After five days of trying to settle in to the Oval Office, President Trump signed an executive order to block federal funding to "sanctuary cities".
There is no legal definition of what a sanctuary city is, but it is used to describe a place that limits how local law enforcement co-operates with federal immigration agents.
The executive order targets jurisdictions that hinder communication with the Department of Homeland Security about a person's immigration status.
The first jurisdiction to do this was Los Angeles, in 1979, where the city's police department forbade officers from detaining people with the objective of finding out their status.
The Immigrant Legal Resource Center found that at least five states - California, Oregon, Vermont, Connecticut and Rhode Island - have laws that limit how much local police co-operate with the feds.
Furthermore, at least 633 counties and 39 cities have policies limiting co-operation, including New York, Washington, Baltimore, Los Angeles and Chicago.
Local authorities cannot stop federal agents - there is no jurisdiction beyond the reach of federal authorities - but they can make their investigations difficult, and make it less likely that an undocumented "alien" will come to their attention.
A Supreme Court ruling in 1997 held that the federal government cannot require state officials to enforce federal law, in a doctrine called the "anti-commandeering principle".
The Washington Post gives the example of an illegal immigrant being arrested for driving without a licence and then their status being discovered in a sanctuary city. They must serve jail time for state charges or pay related fines - after which, they are let go.
The cities do not necessarily offer sanctuary, but leniency. You will not be able to commit a crime, for which you will be rightfully punished, but you will not be punished for your immigration status. Sanctuary cities do not stop federal agents from doing their jobs, but they make it a bit more difficult; a nuisance rather than prevention.