Jeremiah 42:10
If ye will still abide in this land, then will I build you, and not pull you down, and I will plant you, and not pluck you up: for I repent me of the evil that I have done unto you. If ye will still abide in this land, then will I build you, and not pull you down, and I will plant you, and not pluck you up: for I repent me of the evil that I have done unto you.
After Jerusalem's fall the word was spoken to the Jews left as a remnant in the land, coming after the events of chapters 40 and 41 in the reign of Zedekiah, as part of Jeremiah's account of his own experiences including his going into Egypt. The promise came after the people asked for God's guidance and professed they would obey whatever He commanded. They did not sincerely intend to follow it, so the condition tested their desire to go to Egypt.
If they would still abide in Judah rather than fleeing to Egypt, God would build them and plant them rather than pull them down and pluck them up. To build and to plant meant to restore, establish, protect, bless and preserve them. Not to pull down and not to pluck up meant not to destroy and remove them, but to shield them from further destruction and removal. God's statement that He repented of the evil He had done to them meant He would turn and relent from the calamity and disaster He had brought upon them and instead bless them.
If they abandoned the plan to go to Egypt, God promised protection from the king of Babylon and mercy to them. He knew Egypt would soon fall to Babylon, and His purpose was for His people to come under Babylonian rule in a way that would serve their good. Going to Egypt instead would bring sword, famine, and pestilence. They rejected this counsel and went to Egypt, to Tahpanhes, despite Jeremiah's strong protest. This disobedience brought punishment and destruction, followed by later idolatry and the failure of their plans.