The study of how a text came to be placed in a Canon of Scripture. Questions about whether a text should be included in the canon are off-topic here.
For the purposes of this site, please do not ask if a particular text is or ought to be in the canon. Those questions are potentially on-topic at:
Think of canonicity questions as addressing issues not unlike those addressed by the fans of fictional universes. Religions believe their texts form a coherent narrative and so some texts may be included and others excluded. What are the criteria they use? How do those criteria apply to this or that candidate book?
To give an example, The Book of Tobit was not included in the Jewish canon. Why not? What criteria allowed Daniel to become canonical but not Tobit? These are questions of canonicity. It's like solving a jigsaw puzzle with extraneous pieces. For the purposes of this site, we don't care if the picture is correct; only that it's self-consistent.
We prefer questions to consider the canonicity of books according to ancient groups over questions interested in modern religions. This is to avoid continuing controversies. In other words, these questions should focus on the history of how or why various texts came to be included in various canons, not whether or not they are or should be in a canon.