鸿 is a bound form (b.f.) that cannot stand alone as a word; it appears in literary or formal compounds. Its meanings fall into three groups: (1) the literal swan goose (鸿雁), (2) a metaphorical sense of 'letter' or 'correspondence' derived from the ancient practice of using swan geese to carry messages (鸿雁传书, 来鸿), and (3) an abstract sense of 'great' or 'grand' used in auspicious or formal expressions (鸿图, 鸿运). You'll encounter it in classical idioms and formal writing at advanced levels.