Open Education Resources (OER) are online teaching and learning resources freely available for reuse, re-purposing, and sharing.
OER can either be in the public domain, or under a more lax intellectual property license.
OER can be revised, remixed, added upon, translated, and then shared again to meet different needs.
OER can take many forms, such as: syllabi, lesson plans, videos, software, tests, teaching techniques, group activities, writing prompts, textbooks, learning modules, experiments, simulations, and course designs. There are no platform restraints.
Source: The Review Project
There are many reasons instructors might want to use OER:
"Open Educational Resources: What are OER?" by Wellesley College Library is licensed under CC BY 4.0 / A derivative from the original work by University of Illinois Library.