January 1, 2019, all work published in the United States in 1923 entered the public domain. This was the first time work has entered the public domain in twenty years due to copyright extensions.
The public domain includes any work for which copyright has expired or work that has been specifically donated into the public domain by the author/creator. Note that the types of work that could have been copyrighted include: musical scores, musical recordings, writings, movies, artwork, sculptures, graphical works, maps, computer software, architectural plans, etc.
Public domain works are special because you can remix them, sell them or do anything you like with them without first asking for permission from the author/creator (or the current copyright owner). This may inspire creativity, such as remixing!
Because any work published in the US before 1924 is part of the public domain, these works can be found in many places, including your library. There are some places you might wish to go to search for PD works, however.