Open educational resources (OER) are teaching, learning, and research resources that reside in the public domain or have been released under a copyright license that permits anyone to freely use and repurpose them. Open educational resources include full courses, course materials, modules, textbooks, streaming videos, tests, journal articles, and any other tools or materials used to support learning. The real power of OER comes in the '5R permissions' that entitle you to not only access these materials free of charge, but also to make them better:
Source: Lumen Learning