This is a letter in the Bible's New Testament. Paul wrote this letter to Christians in Thessalonica.
He thanked God for their faith, love and hope. They had turned from idols (false gods) to serve God. They were waiting for the return of Jesus. They trusted God in spite of great difficulties.
Paul told them to please God by the way that they lived. They should be holy. They should work hard. And they should love each other. Then he answered their questions about the return of Jesus. He told them not to worry about those who had died. They would not miss Jesus' return. They would meet him before those who were still alive. The return of Jesus will be sudden. Therefore the Christians in Thessalonica must be ready for this. They must continue to work quietly.