What is the Open Game License?
The Open Game License (OGL) is a public copyright license by Wizards of the Coast that may be used by tabletop role-playing game developers to grant permission to modify, copy, and redistribute some of the content designed for their games, notably game mechanics. However, they must share-alike copies and derivative works.
https://en.wikipedia.org/wiki/Open_Game_License
Where can I find the License?
The Open Gaming License can be found at dnd.wizards.com in a document called The Systems Reference Document (SRD), you can download a PDF containing the Open Game Content and the Open Game License by following this link SRD5.