In the United States and Canada, a sanctuary city refers to a municipality that loosens how strongly federal regulations are enforced when it comes to immigration and border control. In Europe, sanctuary city most often refers to a city that welcomes refugees.
In the United States, some places have sanctuary city status written into their laws, while some simply practice loosened regulations in the field. This is the difference between de jure (written) and de facto (practiced) sanctuary status. If a police department has been instructed to pointedly avoid asking about the immigration status of the individuals they encounter, that would be an example of a sanctuary city policy.
There is a sharp divide along political lines on opinion for or against sanctuary cities in the United States. Advocates of sanctuary cities saw that they are a great way to avoid breaking up families and hurting the economy through inhumane deportation practices. Those opposed to sanctuary cities say that observing federal law is the only way we can create comprehensive and humane border control policies, and sanctuary cities make it harder to achieve that end lawfully.
A Washington state immigration lawyer can provide more information regarding sanctuary cities.