The serfs turned over and became their own masters. "The turned-out serf sings" is an idiom, which means that after a slave got rid of the oppression and exploitation of his master, he was finally able to freely express his thoughts and emotions and sing his favorite songs. It is usually used to describe a person who finally gained freedom and liberation after experiencing long-term oppression and suffering, and lived a happy life ever since. This expression is also often used to describe a country or nation that finally achieves freedom, equality and prosperity after experiencing long-term oppression or hardship.