The U.S. Department of the Treasury's Office of Foreign Assets Control (OFAC) has published a notice listing individuals or entities added to the Specially Designated Nationals and Blocked Persons List (SDN List). This means their assets under U.S. jurisdiction are blocked, and American individuals and companies are generally banned from engaging in transactions with them. The notice fulfills applicable legal criteria for sanctions. More information is available on the OFAC website.
Simple Explanation
The U.S. Treasury has named some people or companies that they won't do business with anymore, and now others in the U.S. can't either because their things are on a special blocked list.