What does Isaiah 27:12 mean?
Isaiah concludes this larger section (Isaiah 24—27) with a promise from the Lord. In "that day," meaning the end times (Isaiah 2:11; Ezekiel 30:3; Obadiah 1:15; Acts 2:20; 2 Peter 3:10), the Lord will gather up His people of Israel, one by one. The picture Isaiah uses is of a farmer. The farmer takes harvested grain and processes it to remove what is edible. Others will "glean" the field, meaning they will comb the ground to gather in every bit of good grain which has fallen (Leviticus 9:9–10; Ruth 2:2–3).The Lord will do the same in gathering in the Israelites. The "Brook of Egypt" may refer to the southern border of Israel's rightful territory (Numbers 34:4–5). The space between these two rivers is that of the Promised Land (Genesis 15:18). However, some interpreters think Isaiah's reference is to the Nile River. The following verse refers to people driven into Assyria—represented by the Euphrates—and into Egypt—represented by the Nile (Jeremiah 2:18). This may have been an assurance to the people of Judah that the Lord will not forget those taken into exile in those territories. He will gather up all His lost ones and bring them home.
This theme repeats often in Isaiah's writing (Isaiah 11:12–16 and 35:1–10). The Lord will be faithful to the Israelites all the way to the end, no matter what takes place between now and then.