A license is a document that specifies what can and cannot be done with a work. It grants permissions and states restrictions. Broadly speaking, an open license is one that grants permission to access, re-use and redistribute a work with few or no restrictions (definition from Openedefinition.org). There are many open licenses developed for different areas of knowledge. However, when it comes to open educational resources the most typical and common open licenses used are Creative Commons Licenses. Copyright & Open Licensing FAQs
CC0 "No Rights Reserved" A new CC license created to enable scientists, educators, artists and other creators and owners of copyright or database-protected content to waive those interests in their works and place them completely in the public domain. Others may then freely build upon, enhance and reuse the works for any purpose without restriction under copyright or database law.