Kings in the Ranger's Apprentice universe rule over several countries. During Will's apprenticeship, King Duncan ruled over Auraluen, King Henri over Gallica. King Swyddned was the king of Celtica. Each of the eight kingdoms of Hibernia had a seperate king.
Kings in the RA Universe have essentially the same role as the real medieval kings; to keep order, lead the army into war, work as a judge, and to keep things stable throughout his kingdom along with their Barons or Lords, Shans or Jarls