In Chapter 27, God commands Jeremiah to wear a wooden yoke, symbolizing the yoke of captivity that Babylon will put on Judah by conquering it. However, in Chapter 28, the false prophet Hananiah snaps the yoke off Jeremiah's neck, claiming that Babylon will withdraw and will leave Judah in peace. Wrong, wrong, wrong.
God tells Jeremiah that he'll replace the wooden yoke with a yoke of iron on Judah's neck, so Babylon is going to yoke them even more securely now. Additionally, God says Hananiah will die soon, which he does.
But God also says that he will eventually break the yoke off the neck of Israel, and they won't serve strangers anymore.