The Department of State has announced the addition of certain individuals to a list managed by the Department of Treasury, called the Specially Designated Nationals and Blocked Persons List. This action was taken because these individuals were found to be involved in harmful activities directed by the Russian government. As a result, their property under U.S. jurisdiction is blocked, and U.S. citizens are generally prohibited from doing business with them. More details about these sanctions can be found on the Office of Foreign Asset Control's website.
Simple Explanation
The U.S. government found some people doing bad things for another country and is stopping those people from using their stuff in the U.S. or doing business with U.S. people.