An open textbook is a textbook licensed under an open copyright license, and made available online to be freely used, adapted, and modified by students, teachers and members of the public. They are available for free as online versions, and as low-cost printed versions, should students opt for these.
Many open textbooks are developed through traditional peer review or are vetted by experts in their field. As with any textbook, you are the final judge of whether an open textbook meets the needs of your course.
Source: BCcampus Open Textbook FAQ