Open educational resources (OER) are teaching and learning materials that are freely available to students, educators, and the general public. They are openly licensed so that others can adopt or reuse this material for their own purposes.
To be more specific, when we say that OER are openly licensed, we mean that they can be retained, reused, revised, remixed, and redistributed. In practice this means:
Not all, but many creators of OER choose to apply licenses indicating how others can use their work. Most commonly used are Creative Commons licenses, or a set of statements that indicate the author's wishes when it comes to citing their work, using it for commercial purposes, adapting it, and redistributing it.