What is OER?
"Open Educational Resources (OER), are teaching, learning, and research resources that are free of cost and access barriers, and which also carry legal permission for open use. Generally, this permission is granted by use of an open license (for example, Creative Commons licenses) which allows anyone to freely use, adapt and share the resource—anytime, anywhere." (SPARC)
Check out this graphic explanation of OER from the Georgia State University Libraries.