The
兮 (xī) is a classical exclamation particle with no modern translation.
之 (zhī) here marks possession:
祸之所伏 = 'the place where disaster is crouching.' Together with the first half
祸兮福之所倚, this couplet is the philosophical root of
塞翁失马 ('the old man on the frontier lost his horse — how can you tell if it's bad luck?').
