Open textbooks are part of a larger movement called "Open Educational Resources" (OER)
An open textbook is a textbook with an open copyright license that allows the material to be freely accessed, shared and adapted.
Open licenses allow instructors to adapt, remix, or customize existing open textbooks to maximize instructional content to meet their own learning objectives
Many open textbooks are developed through traditional peer review, others are vetted by experts. As with any textbook, the instructor is the final judge of whether an open textbook meets the needs of the course.