Open Content is a generic name for a creative piece of work (such as text, images, sound and video) which is published under a license which explicitly allows copying (depending on the license chosen), adapting and spreading of that information.
The big advantage of using Open Content material is that you cannot, by accident or on purpose, violate copyrights.
The producers of the original material often use a Creative Commons license to force people to refer to the original material. Sometimes they state other conditions under which the material can or cannot be used e.g. the material cannot be used for commercial purposes. Of course the rules for citing sources also apply to Open Content material.
One specific category of Open Content material is audiovisual extracts. See Remixables for an overview of sources.