What is the meaning of the Prophet's (peace and blessings be upon him) saying: "Whatever you catch, pray, and whatever you miss, complete"?

The meaning of the Hadith is that the latecomer (masbuq) prays with the Imam whatever he catches, and makes up what he missed after the Imam's salām. Scholars differed on whether what the latecomer makes up is the beginning of the prayer or its end. The more preponderant opinion is that it is the end, and that "faqḍū" (make up) means "fa'atimmū" (complete).