A fleet liaison officer was usually a foreign spokesperson who represented the interests of his or her home government's fleet.
During his time on Deep Space 9, Lieutenant Commander Worf served as Starfleet's fleet liaison officer to the Klingon fleet. In 2373, circumstances forced Captain Benjamin Sisko to relieve Worf of his Starfleet duties. As a result, Jadzia Dax was assigned as fleet liaison officer due to her experience with Klingon culture. (DS9: "Soldiers of the Empire")