"Open Educational Resources (OER) are teaching, learning and research materials in any medium – digital or otherwise – that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others with some or no limited restrictions." - UNESCO
OER can be retained, reused, revised, remixed, and redistributed. In practice this means:

"OER is sharing" by Giulia Forsythe is licensed under CC BY 4.0.
Instructors who use or adapt Open Educational Resources and Open Textbooks can save our students who may have difficulty affording textbook prices a great deal of money. OER is a worldwide movement to help foster the free dissemination of information and promote information sharing and equity. But, don't forget to reach out to your librarian for ideas to use fully copyrighted published materials in a class setting, again at no additional cost to students.