Open educational resources are materials designed (usually) by teachers to support teaching and learning. An book may be an OER, a test or worksheet, a presentation, video or simulation, image, sound or any other work. Most who are new to OER have little trouble understanding the educational resource part of OER. Teacher recognize them as valuable, they have used educational resources created by others and educational resources thye created.
The open part of OER can be a little more challenging to understand. For many educators, the most appealing part of OER is that they can be obtained (usually downloaded) and copied and distributed for free. Not all free educational resources are open, however. I can own a resource, allow others to obtain and cop and distribute it, but is may not be open.
Open requires the author allow certain uses. Again, the author is not necessarily allowing all uses, but the author allows some uses. For example, when I release an OER, I typically apply Creative Commons Attribution Non-Commercial 4.0 International license which allows for open use, but those who create derivative works (for example if they translate my work into another language), they cannot sell that derived work.
For several years, “the 5 R’s” have been used to define OER’s: