"If thou wilt test my knowledge from thy writings, 'tis where thy spiritual energy is stored. 'Tis cultivated in thy body like ore within a crucible. It sounds like schools of alchemy that claim the gold produced by the Philosopher's Stone isn't an ore or earthly material, but the means to immortality. But I've never heard of true immortals in my world, and there are immortals who've cultivated golden cores in thine." Claudius keeps a light tone explaining it, but he's also sparing Lan Wangji the effort of explaining it over again. If what Lan Wangji has to say weighs heavy, Claudius won't have him weighing more words than necessary.