This concept refers to the state of authorship of a particular work. Works that fall into the public domain belong to the public as a whole and are not protected by the copyright law.
"The public domain is not a place. A work of authorship is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner."
Source: Copyright.gov Definitions
Copyright Term and the Public Domain in the United States (This chart is maintained and updated by Cornell University Library and provided for your convenience. UMMC does not take responsibility for content.)