What does Jeremiah 18:12 mean?

At the instant I speak concerning a nation, and concerning a kingdom, to build and to plant it;

if they do that which is evil in my sight, that they not obey my voice, then I will repent of the good with which I said I would benefit them.

“Now therefore, speak to the men of Judah, and to the inhabitants of Jerusalem, saying, ‘Yahweh says: “Behold, I frame evil against you, and devise a plan against you. Everyone return from his evil way now, and amend your ways and your doings.”’

Therefore Yahweh says: “Ask now among the nations, ‘Who has heard such things?’ The virgin of Israel has done a very horrible thing.

Will the snow of Lebanon fail from the rock of the field? Will the cold waters that flow down from afar be dried up?

For my people have forgotten me. They have burned incense to false gods. They have been made to stumble in their ways, in the ancient paths, to walk in byways, in a way not built up;