The Department of State has added one or more individuals to the Treasury's Specially Designated Nationals and Blocked Persons List because they meet the legal criteria set by an Executive Order targeting harmful activities by the Russian government. As a result, their assets in the U.S. are frozen, and American citizens are generally not allowed to do business with them. For more information, individuals can check the SDN List on the Office of Foreign Asset Control's website.
Simple Explanation
The Department of State has made a decision to stop certain people from doing business in the U.S. because they did things that were not nice, like harmful activities related to the Russian government. Now, these people can't use their money or things in America, and U.S. folks aren't supposed to deal with them.