Condition refers to both the book's external physical appearance and to the completeness of its contents. A book that has been rebound or is in less than fine condition must be very important or in high demand to be of substantial value. Another term that you hear with fine books is rare books. Usually several factors are present including the intrinsic importance of the work, its scarcity, and collectors' interest in it. A rare book must be in good condition to make it valuable.
Condition grading is a subjective process.
Even among experienced booksellers there are differences of opinion. However, there are general standards and terms which can help you. Or seek out experts in the field. Books are a fusion of the intellectual and the artistic. They can be appreciated for their literary content and as objects, as works of art themselves.