OER stands for Open Educational Resources. "Open Educational Resources are teaching and learning materials that you may freely use and reuse, without charge. OER often have a Creative Commons or GNU license that state specifically how the material may be used, reused, adapted, and shared”). OER materials can include textbooks, test materials, instructional videos, and other learning materials and tools.
OER might be similar to what you are currently doing. If you have ever used a worksheet that another instructor created, that's the basic concept of OER. Fundamentally, the only difference between OER and the worksheet you got from a colleague (or vice versa) is that the open license gives you and others formal permission to use, adapt and share it.
"Open Educational Resources (OER) are any type of educational materials that are in the public domain or introduced with an open license. The nature of these open materials means that anyone can legally and freely copy, use, adapt, and re-share them. OERs range from textbooks to curricula, syllabi, lecture notes, assignments, tests, projects, audio, video, and animation."