What does Isaiah 21:10 mean?

In the end, the purpose of the oracles is to motivate God's people to put their hope in Him alone. The might and power of these other nations will only fail. Judah was not the only nation holding out hope that Babylon's defiance of Assyria would protect them from invading Assyrian armies. The Lord did not want Judah to seek help from other sources. He wanted them to depend on Him alone for salvation from all that threatened them.

The prophet concludes this short oracle by calling Judah his "threshed and winnowed one." This farming phrase was sometimes used to describe those who had been judged by the Lord and endured much. A further breakdown of this phrase would be farmers used animals, sometimes pulling large carts, to trample the harvested grain. Crushing the grain in this manner separated it from the worthless chaff or husks. Then the grain would be tossed in the air so the chaff would blow away leaving the true grain to fall back to the earth. Judah has also gone through such a process.

The message here is either that Judah will experience more of this painful threshing, or that they had already experienced it. The good news is that after the process was over only the best "grain" remained. Although it would have hurt at the time to lose the hope that Babylon might save them from Assyria, it cleared away the "chaff." Babylon's destruction at the hands of Assyrians removed an "option" that the people of Judah had to save them. When the destroyer of Babylon later had Jerusalem surrounded and under siege, the Lord demonstrated His power to save His people without any outside help (Isaiah 37:33–38).

What Isaiah has been revealing to the people of Judah is exactly as it was revealed to him by the Lord of hosts. These formal names for the Lord show just how serious Isaiah's revelation is.