Albert Barnes Commentary Jeremiah 33:9

"And [this city] shall be to me for a name of joy, for a praise and for a glory, before all the nations of the earth, which shall hear all the good that I do unto them, and shall fear and tremble for all the good and for all the peace that I procure unto it." — Jeremiah 33:9 (ASV)

It - The city, Jerusalem.

They shall fear and tremble - With terror, because of the eternal opposition between right and wrong, truth and error. The nations of the earth as opposed to Israel represent the world as opposed to the Church.