This term is a somewhat old form of describing a contract to carry goods by sea, that is a “contract of carriage“. Such a contract is now more correctly and simply referred to as a “charter”. “Affreightment”, comes from French and means “the hiring of a ship to carry cargo”.
However, in modern shipping terminology, a “Contract of Affreightment” (COA) has a specialised meaning. It is now a contract to move a (large) volume of cargo over an agreed period of time. Examples are VOLCOA and INTERCOA 80.
Under the Hague Rules or Hague-Visby Rules, a “contract of carriage” also means a bill of lading or any similar document of tile insofar as such document relates to the carriage of goods by sea.