Creative Commons ("CC") licenses are standards for published works that aim to offer a "free, simple, and standardized way to grant copyright permissions for creative and academic works; ensure proper attribution; and allow others to copy, distribute, and make use of those works," according to the Creative Commons website.
CC license are an alternative to traditional copyright which inhibits the sharing and reuse of materials. When reusing others' OER, it is important to pay attention to the licensing to understand how you may make use of it. Applying a Creative Commons license to your own OER that allows for the reuse and remixing of your work is an important component of open education. It's what makes it "open!"