en_tn/isa/27/12.md

It will come about

This phrase marks an important event that will happen.

on that day

"at that time"

Yahweh will thresh

Yahweh gathering his people to bring them back from the foreign nations to the land of Israel is spoken of as if he were threshing wheat to separate the grain from the chaff. (See: rc://en/ta/man/jit/figs-metaphor)

from the Euphrates River to the Brook of Egypt

Isaiah mentions the Euphrates River and the Wadi of Egypt to mean that Yahweh will bring back the people of Israel who were exiled in lands near those waters, that is, Assyria and Egypt. The Euphrates River is northeast of Israel, and the Brook of Egypt is southwest of Israel.

you ... will be gathered together one by one

This can be stated in active form. Alternate translation: "Yahweh will gather you together one by one" (See: rc://en/ta/man/jit/figs-activepassive)