You may be using Open or Affordable course material already, without realizing that you're helping Adams State's campus OER Initiative!
OER is often thought to refer to online educational resources. While most OER are digital, the important distinction is that not all digital material is open.
"Open" refers to the licensing. Instead of traditional copyright ensuring "All Rights Reserved," open licenses like Creative Commons provide "some rights reserved," while allowing for remix and redistribution. See the "Licensing" tab for more information.
OER is likewise assumed to refer to free* textbooks, fairly exclusively. However, as with any "educational material," there's far more to it than that! As traditional publishers try to entice faculty with text/quizzes and answer banks, slide-decks, Audio-visual material, and other pre-developed course content, so OER works to provide those same resources.
*Technically "open" does not equal "free" -- there are still economic dimensions to consider, such as time commitments of contributors, resources, and uncalculated monies. Some "Open" content providers like the Open Textbook Network's Open Textbook Library (OTN, OTL) require paid subscriptions. While it could certainly be argued that payment defeats the purpose of "open", that's the way it is for now.
For the University:
Hannans, J., et. al. Closing the gap with OER: Textbook affordability and student success for historically underserved students [Google slides]. Webinar presentation based on data published in openCI White Paper [PDF document] available from https://www.csuci.edu/tli/openci/openci-white-paper.pdf