Creative Commons (CC) licenses are a way that copyright owners allow others to use and sometimes remix their work without first asking permission. Works with a CC license allow re-use of the work under certain conditions depending on the license chosen by the copyright holder.
Watch this 3 minute video to understand Creative Commons.
Creative Commons licenses indicate what the copyright owner expects from people re-using their work through the specific terms of each license. Review this Creative Commons "About our licenses" information page for more information. In particular, read the details of all six licenses listed in the second half of this page.
When creating a new work that will incorporate other images or audio, searching for Creative Commons or similarly licensed works that meet that need is an easy way to ensure you're in compliance with copyright law. Creative Commons hosts a Creative Commons Search page that links to multiple search engines that include Creative Commons-licensed material, but licensed content can be found in many common search engines including Google (Google Search Help page on searching by usage rights).
Why do we need a system to allow content creators to license their works in the way that Creative Commons licenses allow?