Open educational materials have an "open" license.
Licenses are a way of giving permission to someone to use your work in a certain way. (When you hear "license," think "permission.") You've probably seen Creative Commons licenses in educational settings. Similar to open source software licenses, they allow the creator (or rights holder, if copyright has been transferred) of an educational or scholarly work to retain their copyright while granting users permission to use their work in certain ways. All the creator needs to do is share the license on the work itself.
Depending on the license chosen by the creator, users may be allowed to:
Open licenses are less restrictive than "all rights reserved" copyright, where a creator does not specify any type of license, but more restrictive than works in the public domain, where copyright has expired, or where a rights holder has waived all rights to their work.
Defining the "Open" in Open Content and Open Educational Resources was written by David Wiley and published freely under a Creative Commons Attribution 4.0 license at http://opencontent.org/definition/.
For content that you create, consider making it more open and available for more people to use and reuse.
Creative Commons licenses are a simple, standardized way to give others permission to share and use your work without others having to ask your permission -- on conditions of your choice.