From The Washington Post:
The Supreme Court on Monday struck down several key parts of Arizona’s tough law on illegal immigrants, but it left standing a controversial provision requiring police to check the immigration status of people they detain and suspect to be in the country illegally.
SCOTUSblog provides an explanation “in plain English”:
The decision was largely (but not entirely) a victory for the federal government: the Court held that three of the four provisions of the law at issue in the case cannot go into effect at all because they are “preempted,” or trumped, by federal immigration laws. And while the Court allowed one provision – which requires police officers to check the immigration status of anyone whom they detain or arrest before they release that person – to go into effect, even here it left open the possibility that this provision would eventually be held unconstitutional if not applied narrowly in Arizona.