What Does Public Domain Software Mean?
Public domain software is any software that has no legal, copyright or editing restrictions associated with it. It is free and open-source software that can be publicly modified, distributed or sold without any restrictions. SQLite, I2P and CERN httpd are popular examples of public domain software.
Techopedia Explains Public Domain Software
Public domain software has no ownership and is available for use, modification and commercialization by anyone. Typically, public domain software is intentionally or voluntarily uncopyrighted, unpatented and is unrestricted by its developer/author. It is different from free software and freeware that does has copyrights and patents associated with it.
Although there are no licensing requirements with public domain software, The Unlicense, Creative Commons License and WTFPL are based on a similar approach.