Open Educational Resources, often referred to as OER, are educational materials that are created with the intention of free sharing. An OER will be marked with a Creative Commons license that tells teachers they are free to use the materials with their students. In some cases, the license will also allow the teacher to alter the resource to create a different version.
To learn more, check out the YouTube video and set of links at the bottom of this page.
This page also links to sources of free curriculum materials that might not be marked with a Creative Commons license, so not technically OERs.