What does Isaiah 6:12 mean?

This direct revelation from the Lord is what Isaiah is to preach to the people of Judah. This passage also explained how the people will respond to his preaching (Isaiah 6:9–10). In short, Isaiah is to warn the people about the coming judgment of God. The people will not understand and will refuse to repent from their sinfulness. Even so, God will not allow them any excuses that they were not warned.

Isaiah has asked the Lord "how long?". Presumably, he means how long must he continue to preach to the unresponsive people of Judah. The Lord's answer has been that Isaiah must keep preaching until the judgment comes and the land is laid to waste (Isaiah 6:11).

The Lord continues his instruction that Isaiah must keep preaching until the Lord removes the people from the land and leaves it in ruin. This is what happens, of course, when the Assyrians and then the Babylonians carry off the people of Israel and then later Judah into exile (2 Kings 24:14). That alone will not necessarily be the end of the Lord's judgment if too many people remain in the land (Isaiah 6:13).