Copyrights exist in order to protect authors of documentation or software from unauthorized copying or selling of their work. A copyright infers that only with the author’s permission may such activities take place.
A Copyleft, on the other hand, provides a method for software or documentation to be modified, and distributed back to the community, provided it remains Libre.
In the case of Libre Documentation, an author can place his or her copyright into the document, and use distribution terms, such as those in the GNU Free Documentation License, which gives everyone the rights to use, modify, and redistribute the code, but only if those distribution terms remain unchanged. This ensures that the source code and the freedoms are legally inseparable. This is known as “copyleft”.
If a program or document was uncopyrighted and in the public domain, changes could be made and the program or document could be re-distributed as a proprietary product. The copyleft ensures that not only is the original source free, but that all modifications must be made free, and permission is granted for all who follow in modifying that same program or document, provided they abide by these terms.
Applying a free software or free documentation license to an application or document qualifies the product as Libre, and protects the open source community at large from it becoming commercial or proprietary.