Licenses are legal agreement that grants certain permissions to the licensee from the licensor. In the context of copyright, a copyright owner can grant a license to another person to use their work in certain ways, and under certain terms.
Unless the owner of the work relinquishes their entire copyright to the work, they still retain rights to it and control over it, subject to whatever terms they have already established.
There are several licensing options and organizations that simplify the process of authorizing specific use of a work, so that the creator of the work can set broad use permissions without having to negotiate specifics with anyone who wants to use their work.
The most familiar license option—and the most relevant for text and visual content—is Creative Commons, which you can read more about below. Remember there are several others including "copyleft" licenses such as the GNU GPL (a software license), the Free Art License, and more.
Creative Commons is an organization that has created a set of easy to use and understand copyright licenses that creators can attach to their works. These licenses specify certain terms under which other individuals can reuse, remix, and distribute the original author's creation, while preserving author rights to a specified extent.
The image below provides a comparison of the terms and levels of freedom granted by the various Creative Commons licenses, in comparison with public domain licensing and traditional, all-rights-reserved copyright.