Open Educational Resources (OER) are teaching, learning, and research resources released under an open license that permits their free use and repurposing by others.
OER can be textbooks, full courses, lesson plans, videos, tests, software, or any other tool, material, or technique that supports access to knowledge. (SPARC Open Education Fact Sheet)
When researching OER, you may come across references to the following 5R activities described by David Wiley of Lumen Learning:
The terms "open content" and "open educational resources" describe any copyrightable work (traditionally excluding software, which is described by other terms like "open source") that is licensed in a manner that provides users with free and perpetual permission to engage in the 5R activities:
This material was created by David Wiley and published freely under a Creative Commons Attribution 4.0 license at http://opencontent.org/definition/.
It's likely the OER you decide to use will be made available under a Creative Commons license. Check out the Creative Commons tab for more information about specific license terms.