It is the government's appeal of a ruling by the U.S. Court of International Trade, which found that President Donald Trump's unilateral imposition of tariffs on certain goods entering the United States from certain foreign countries is unconstitutional.

The lower court recognized that the Constitution established the separation of powers and gave the taxing power exclusively to Congress.

Here is the backstory.

The Constitution's structure separates powers among the three branches and articulates their core responsibilities. Congress writes the laws in areas of governance delegated to it. The president enforces the laws faithfully. The courts interpret the laws and the Constitution. Unlike the congressional and presidential powers, however, the judicial powers do not appear in the document. Rather, they were self-pronounced by the Supreme Court as integral to the judicial function.