A so-called "sanctuary city" is one that refuses to cooperate with federal law enforcement. 

Specifically, when the city arrests an illegal alien who is wanted for breaking the law in some other state, or merely for violating a deportation order, federal agents issue a "detainer," asking the city to hold the illegal for 24 hours, until the feds can come get him.

In sanctuary cities the mayor has directed the cops not to cooperate, so they release the illegal right away.  He (most are male) vanishes back into the general population, usually to be arrested again for committing another crime.

Los Angeles has been a "sanctuary city" even before California's totally Democrat-controlled legislature passed a LAW barring any government employee or private business from cooperating with the federal government regarding gaining custody of an illegal.

Recently a conservative outfit asked LA mayor Gil Garcetti if he worries about potentially facing federal charges. Here's what he said:

Not at all. I think it’s a political circus. I think it’s about stirring up, you know, blowing a dog whistle – not about anything real. There’s not a single document we’ve ever cut back, and they know that. There’s not a single city in America that has done that, so it was kind of a weak and pathetic move.

Consider the sentence bold-faced above:  What does he mean by "There's not a single document we've ever cut back"?  Let me help you:  It's bullshit--designed to make the listener believe the guy answered the question in a meaningful way. 

The statement that "There's not a single document we've ever cut back" has nothing to do with the charge that they are refusing to cooperate with federal law.  And of course Garcetti knows this.  He knows that if he admits the truth--that California (as well as LA) is refusing to cooperate to help the feds remove people illegally in the U.S.--it stirs up conservative Americans--who, amazingly, don't like it when illegals murder their kids or spouses, or when members of the thuggish MS-13 gang terrorize and murder, or when illegals driving with three or four times the legal blood-alcohol limit kill an entire American family.

So Democrat Garcetti will go to great lengths to avoid admitting the truth--at least when he's not in front of a friendly audience of Mexican supporters.  There he'll brag about how his policies have protected all the "undocumented" people here (Demspeak for illegals).  But outside those venues he's all about gobble-speak:  Say words that don't actually mean anything but sound like a firm denial of any wrongdoing.

If I were Trump I'd direct federal marshalls to arrest every member of the California legislature, and Garcetti and Jerry Brown and the mayor of San Francisco.  Put 'em in a federal prison until trial.  No bail.  Then repeat as needed.  We need to see if the states have to comply with federal law.