Imagine that I've pasted an image of a UFO here from OpenClipArt.org. That image is free to be used, remixed, and shared because it is licensed under Creative Commons. Creative Commons licenses helps content creators to share their work more freely than copyright allows. Because the UFO is a Creative Commons image, it is acceptable to edit and remix it to create and share new images (note that it contains no restrictions on use).
Copyright law protects a work the moment it is put into a fixed form (so the moment the words are written, the video is recorded, or a picture is snapped) and states that there are certain rights with regards to the work that only the creator holds. For example only the creator may reproduce the work (but see fair use and the other copyright exceptions, like face-to-face teaching). Fortunately, some authors and creators are happy and willing to share their work more freely than U.S. copyright law currently allows. In order to make their wishes clear to both you and the law, they often license their work. A license details the terms and conditions the author has established with regards to using his or her works. There are many different types of licenses but some of the most common and useful in an academic setting are Creative Commons Licenses.