OER is short for Open Educational Resources.
They are FREE resources!
OER are teaching and learning materials that are released under an open license, which allows them to be freely used and reused at no cost. Unlike copyrighted resources, OER have been created by an individual or organization that chooses to retain few, if any, ownership rights. — OER Commons
OER can be "textbooks, full courses, lesson plans, videos, tests, software, or any other tool, material, or technique that supports access to knowledge" — SPARC Open Education Fact Sheet (pdf)