The xiù reading is the technical term for the 28 "lunar mansions" of traditional Chinese astronomy — divisions of the sky that mark where the moon sits on each of its 28 days. Each mansion has a name and a constellation, and they group into four cardinal Symbols:
青龙 (East/Azure Dragon),
白虎 (West/White Tiger),
朱雀 (South/Vermilion Bird),
玄武 (North/Black Tortoise). You'll meet xiù in fantasy novels, wuxia, and any text discussing classical astronomy.