According to the William and Flora Hewlett Foundation, "OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. Open educational resources include full courses, course materials, modules, textbooks, streaming videos, tests, software, and any other tools, materials, or techniques used to support access to knowledge."
In short, Open Educational Resources (OER) are things like open-access textbooks, lesson plans, course materials, and more. They are available in a variety of formats and can easily be mixed and matched to create a unique course experience.