OER stands for open educational resources. OER are resources that are openly licensed which means they are both free to use, and open for revision without having to ask the original author for permission. OER are often used in zero textbook cost (ZTC) courses instead of traditional textbooks; this is one of the reasons a course may be designated ZTC as you are not be required to purchase an expensive textbook.
When people refer to OER they are talking specifically about resources that permit one or all of what are known as the 5 R's: retain, reuse, revise, remix, or redistribute.
Here's what the 5 R's mean:
Check out this video for a brief overview of why open education matters:
Want to know more about what your instructors are doing with OER? Check out the faculty OER site for more information.