Paul and Silas were imprisoned because they were preaching the gospel. They were proclaiming Jesus as the Messiah and telling people to turn from their sin and believe in him. The authorities didn't like this because it was disruptive and challenged their power. So they had Paul and Silas arrested and put them in jail. But even in jail, they continued to praise God and share the gospel with anyone who would listen. And as a result, many people came to faith in Jesus Christ!