What does it mean that a book was decided to be or not a part of the Nac"h?
There are several answers to this question. Some, but not all, would include
Whether it defiles the hands (Yadayim 3:6)
If one is required to save the book from the fire (Shabbat 115)
If you can ask questions from one book to another, such as contradictions in practice (for an ...
To answer your question first requires understanding what the English word, canonization means and its root, canon.
It's actually from the Latin and is generally associated with the Catholic Church. To canonize, the verb form, has essentially three meanings all of which relate to consensus of agreement.
1) It is a process of forming a consensus to declare ...