皂 is a bound form (b.f.) — it cannot stand alone as a word and always appears as part of a compound. Its main modern meaning is 'soap', seen in the common word 肥皂 (fèizào, soap). It also carries a classical literary sense of 'black', found in compounds like 皂白 (zàobái, black and white, i.e. right and wrong). Two less common historical/technical senses are 'yamen runner' (皂隶, zàolì) and 'honey locust' (皂角, zàojiǎo).