Many OERs use Creative Commons Licenses to communicate just how "open" the resource is.
Copyright law grants, by default, "all rights reserved" to authors (or other copyright holders) to protect their claim to a work and profits generated from it.
Creative Commons is a popular way for copyright holders to modify these rights to allow others to reuse, modify, distribute, or even profit from their works without asking permission. The works are still copyrighted and must be cited when used as an information source in a research paper, but the author has opted to allow others to use the work within selected restrictions.
The particular combination of restrictions is selected by the copyright holder and is usually represented in code and/or image. For example,
This license specifies that you may modify, distribute, and reuse the work as long as you give attribution (credit) to the original author and you use the work non-commercially.