The U.S. Department of State has published a list of entities and subentities associated with Cuba, known as the "Cuba Restricted List." This list includes organizations connected to the Cuban military, intelligence, or security services, with which direct financial transactions are generally prohibited due to Cuban Assets Control Regulations (CACR). Transactions with these entities are seen as disproportionately benefiting the Cuban government while impacting its people and private enterprises negatively. The list is available on the State Department's website and will be updated periodically.
Simple Explanation
The U.S. Department of State has made a list of groups and companies in Cuba that Americans aren't allowed to send money to or do business with because they help the Cuban government in ways the U.S. doesn't like.