Open textbooks are books covered by an open copyright license and availabe freely to students, teachers and members of the public. Often, though it depends on the specific license, anyone will have the right to access, download, revise, or customize the content.
For more information, see Educause Learning Initiative's "7 Things You Should Know About(TM) Open Textbook Publishing"
Image: "Free the textbook" by Libby Levi, opensource.com is licensed under CC BY-SA 2.0