A work in the public domain is not protected by copyright. It may never have been protected (such as a work of the federal Government), or protection may have expired. Materials in the public domain...
Works can arrive in the public domain in a number of ways: 1) Copyright can expire at the end of its natural term 2) Copyright was improperly or not renewed 3) Copyright was abrogated (voluntarily...
It means that that piece of work is freely usable by any member of the public without having to pay royalties and/or fear of being the subject of a (non-frivolous) patent, trademark or copyright...