Creative Commons is a global nonprofit that focuses on expanding the range of copyrighted works that are available for others to build upon legally, without having to obtain a direct license from the copyright holder first. The non-profit provides free legal tools to help content creators who are interested in encouraging the reuse of their works by assisting them in ways they can offer these works for use to the general public under standardized terms.
Creative Commons has released a number of copyright-licenses (also known as Creative Commons licenses) that are free of charge to the general public. The various licenses give everyone a simple, standardized way to grant copyright permissions to their creative works. You can read more about the various Creative Commons licenses and how to obtain them here.