According to the Hewlett Foundation, "OERs 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 2019, the Textbook Affordability Committee surveyed students and faculty at ISU and found that 73% of students did not acquire a textbook or course material due to cost, and 89% of students delayed purchasing textbooks or materials because of their cost. OERs remove that barrier, allowing students to come prepared to class on day one.
Additionally, the ability to remix OERs means that you as an instructor can create the best package of course materials for your students. You can customize and update materials as needed, within the bounds of copyright.
(borrowed from Milner's Affordable Course Materials Guide, another wonderful resource for these materials)