The public domain is the pool of creative works that are not owned or controlled by anyone (anymore) - stuff that belongs to everyone.
Because in many jurisdictions intellectual property expires 70 years after death of the author lots of material gets in the public domain this way.
Some projects have original content contributed entering the Public Domain unless the author explicitly states otherwise. This concept is called Primarily Public Domain (PPD), which has been extended to the concept of Ethical Public Domain.
In recent years the time it takes for works to become public domain has been extended, in such a way that works created after the 1920s will actually never become public domain. These extensions happen to coincide with Mickey Mouse's birth date. Lawrence Lessig was involved in a court case to prevent the extension of the public domain and lost. Consequently he started the Creative Commons organization and licenses.