In the days of King Jehoiakim, a word comes to Jeremiah from God. It tells him to go see the Rechabites, bring them to the temple, and offer them some wine.
So Jeremiah takes the Rechabites to the chambers of a man of God named Hanan in the Temple, and offers them wine.
They refuse because their ancestor, Jonadab, told them not to ever drink wine. He also told them never to plant a vineyard, build houses, or settle down and practice agriculture. They're supposed to live a nomadic existence in tents.
The Rechabites are only living in Jerusalem because of Nebuchadnezzar's invasion, so they're stuck in one place for now.
God speaks to Jeremiah again and tells him to go to the people of Judah. He's supposed to tell them to learn a lesson from the Rechabites.
The Rechabites continue to obey their ancestor's laws, but the people of Judah can't obey God even when he commands them right to their faces.
So in punishment, God's going to bring disaster on Judah (somewhat predictably, at this point).
But God promises the Rechabites that they'll always have God's favor.