When a work is no longer restricted under copyright law it goes into the public domain. This happens naturally when copyright runs out but with CC licenses a work can also be specifically dedicated to the public domain.
In the public domain there are no restrictions for use - anyone can do anything they want with it. Examples of public domain works are the writings of Edgar Allen Poe or the musical scores of Chopin. Specific recordings of Chopin's music however maybe copyrighted by the musician(s) recording them.
Learn more about public domain and Creative Commons at: https://creativecommons.org/share-your-work/public-domain/