One Piece is a manga/anime series about pirates written by Eiichiro Oda that started in about 1997.
If you're referring to the titular treasure, then that is a much more difficult question to answer. All we know is that it was hidden on the island of Raftel, the last island on the Grand Line, and that it was hidden there by the King of the Pirates Gol D. Roger. As to what it actually is, we have no idea. Since each character has their own interpretation of what treasure is, it could just be something that Roger was very fond of.