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.
Related Commentaries of Jeremiah 33:9
Jeremiah 33:9
9 And it shall be to me a name of joy, a praise and an honour before all the nations of the earth, which shall hear all the good that I do unto them: and they shall fear and tremble for all the goodness and for all the prosperity that I procure unto it.