From OER Commons: "Open Educational Resources (OER) are teaching and learning materials that you may freely use and reuse, without charge. That means they have been authored or created by an individual or organization that chooses to retain few, if any, ownership rights. For some of these resources, that means you can download the resource and share it with colleagues and students. For others, it may be that you can download a resource, edit it in some way, and then re-post it as a remixed work. OER often have a Creative Commons or GNU license that state specifically how the material may be used, reused, adapted, and shared." (What is OER Commons?)
Many of these resources are licensed under Creative Commons licenses, which allow users varying degrees of freedom to reuse, adapt, and build upon content.
You can learn more about what each license allows at the Creative Commons website.