The Jones Act, when used in reference to the Merchant Marine Act of 1920, prohibits the transportation of goods or people between U.S. ports by vessels built outside of the U.S. The Act also requires that these vessels be owned, operated, and manned by U.S. citizens. While the Jones Act has been credited with improving national security and nurturing the domestic commercial shipping market, it has fallen under much criticism for creating uncompetitive markets and high prices.
You can read about its other provisions at its Wikipedia article.
