Copyright is a set of protections for creative works, given to the authors of the works for a limited time (generally the lifetime of the author plus 70 years). Copyright protects authors' interests and encourages new creativity by providing the author control over things like using, distributing, or remixing the works.
Once these protections expire, or are expressly waived by the authors, works enter the Public Domain. Works in the Public Domain may be used by anyone for any purpose, with no attribution necessary.