Copyleft is (obviously and as a typical example of hacker humor) a pun on the word copyright. It is a legal instrument based on copyright that makes sure the copylefted material stays in the commons, available for the general public and cannot be appropriated by private interests. One could say it's like public domain but with an insurance. However, unlike public domain, copylefted material remains under copyright and derivative versions can only be made if they are similarly licensed. This means no additional restrictions can be placed on the material. Not following these rules constitutes a copyright infringement.
Copyleft was designed and developed by Richard Stallman and the Free Software Foundation in the 1980s. The first popular license to embed copyleft was the GNU GPL 1.0 in 1989.
A similar construct can be seen in the Creative Commons Share Alike licensing provision.