"Sadko" is a Russian epic poem that tells the story of a merchant and musician named Sadko. He travels to the underwater kingdom of the Sea King and participates in a contest of music. His adventures highlight themes of bravery, love, and the pursuit of fortune. The poem is part of the Russian folklore tradition and has been adapted into various art forms, including opera and ballet. It reflects the rich cultural heritage of Russia and showcases the importance of music and storytelling in its history.