‘And Yahweh will be known to Egypt, and the Egyptians will know Yahweh in that day. Yes they will worship with sacrifice and oblation, and will vow a vow to Yahweh and will perform it.'
The essence of this is conversion to Yahweh, leaving behind their own gods. They will know and acknowledge Yahweh. They will come before Him in worship and praise. They will be committed to Him and vow to Him and perform their vows. Their lives will be wholly His.
No doubt Isaiah's hearers were incredulous. The thought of Egyptians turning from their idols to serve the living God would have seemed incredible. And yet it did happen. For in the early centuries AD there was widespread conversion to the Jewish but universal Scriptures and to a Jewish but universal Messiah as Egyptians responded to the call of Christ, and the Alexandrine church became famous throughout the world. And a Christian church has survived there through all the trials of the centuries and will emerge triumphant at last, for among them are God's chosen remnant who represent the whole.