An OER is an open educational resource. This can be anything from full textbooks to modules to syllabi to test banks to images to videos, and many more.
What's an open educational resource? Free + permissions.
The Hewlett Foundation defines them like this: Open Educational Resources (OER) are teaching, learning, and research materials that are either (a) in the public domain or (b) licensed in a manner that provides everyone with free and perpetual permission to engage in the 5R activities [retain, reuse, remix, revise, and redistribute].
Library-licensed resources are resources that the library has purchased that are free for your students to use. You'll want to look for resources that are DRM-free, which means students can download, print, and read as much as they want with no technology restrictions. Another good option is ebooks with unlimited users, because that means that all your students could access the book at once, and usually print a good chunk of the book at a time if desired.