Open Educational Resources or in short OER are teaching, learning and research materials in any medium – digital or otherwise.
The main requirement is that the material has been released under the terms of an open license, e.g. Creative Commons, which allows it to be freely used for educational purposes. Using your OA-publication in a course you teach is one example, but OER can be any type of educational material: their scale varies from something as small as a class handout or image to something as large as a textbook or online course.