WASHINGTON — When President Trump on Wednesday signed a bill that curtails his ability to lift sanctions against Russia, he issued a signing statement challenging the constitutionality of several provisions in the new law that restrict his executive powers.

“While I favor tough measures to punish and deter aggressive and destabilizing behavior by Iran, North Korea, and Russia, this legislation is significantly flawed,” he said. “In its haste to pass this legislation, the Congress included a number of clearly unconstitutional provisions.”

Here is what you need to know.

What is a signing statement?

A signing statement is a document issued by a president when he signs a bill into law. It contains his legal interpretation of statutes created by the bill, often instructing subordinates in the executive branch to interpret them the same way. The device becomes controversial when presidents use it to claim that new laws are unconstitutional intrusions into their executive powers, meaning they can lawfully bypass such limits.

Signing statements were once rare but became more common starting with the Reagan administration. George W. Bush, whose administration embraced sweeping views of executive power, used them to challenge more provisions than all previous presidents combined, including a torture ban. Barack Obama criticized Mr. Bush’s practice as an abuse during the 2008 campaign, but as president he sometimes issued them, too.