A more thorough explanation:

Arizona v. United States was a Supreme Court case that dealt with Arizona's Support Our Law Enforcement and Safe Neighborhoods Act (S.B. 1070). The case held that certain provisions of the law were preempted by federal law, while one provision needed to be allowed to be practiced before deciding whether it should be stopped.

In 2010, Arizona passed S.B. 1070 to discourage and deter the unlawful entry and presence of aliens and economic activity by persons unlawfully present in the United States. The law had four provisions:

• Section 3 made it a state misdemeanor to fail to comply with federal alien-registration requirements.
• Section 5(C) made it a state misdemeanor for an unauthorized alien to look for or perform work in Arizona.
• Section 6 authorized officers to execute warrantless arrests where “the officer has probable cause to believe . . . [a person] has committed any public offense that makes the person removable from the United States”.
• Section 2(B) required officers in certain scenarios, when conducting a stop, detention, or arrest, to make efforts to verify the person’s immigration status with the Federal Government.

The Supreme Court held that sections 3, 5(C), and 6 of S.B. 1070 were preempted by federal law, but section 2(B) must be allowed to be construed in practice before deciding whether the provision should be enjoined.

The Court found that the federal government has broad power to regulate immigration, and state laws that conflict with federal law are preempted. The Court held that sections 3, 5(C), and 6 conflicted with federal law and were therefore preempted. However, the Court held that section 2(B) was not necessarily preempted by federal law, and left it to the state courts to determine whether it conflicts with federal immigration law.

Section 3 of S.B. 1070 made it a state misdemeanor to fail to comply with federal alien-registration requirements. This provision was found to be preempted by federal law because it created a state misdemeanor for conduct already proscribed by federal law, thereby infringing upon the federal scheme that regulates the field of alien registration.

Section 5(C) of S.B. 1070 made it a state misdemeanor for an unauthorized alien to look for or perform work in Arizona. This provision was found to be preempted due to Congress’ enactment in 1986 of the Immigration Reform and Control Act, which specifically addressed the employment of illegal aliens.

Section 6 of S.B. 1070 authorized officers to execute warrantless arrests where “the officer has probable cause to believe . . . [a person] has committed any public offense that makes the person removable from the United States”. This provision was found to be preempted as well, as an impediment to the removal system created by Congress.

Section 2(B) of S.B. 1070 required officers in certain scenarios, when conducting a stop, detention, or arrest, to make efforts to verify the person’s immigration status with the Federal Government. This provision was not necessarily preempted by federal law according to the Court, but needed to be allowed to be practiced before deciding whether it should be stopped.