What is a critical or explanatory note that accompanies a literary work called?

The term being described is commonly referred to as "annotation." Annotations involve notes that clarify, explain, or provide additional information regarding a literary work, often including personal reflections or insights on the text. Annotations are typically placed in the margins or in a separate document and serve to enhance the reader’s understanding of the work by highlighting key themes, historical context, or literary devices used by the author.

Footnotes, while also a form of explanatory notes, are usually placed at the bottom of the page and are often used for citations or references rather than personal commentary. Commentary can refer to broader analysis or discussion about a text but doesn't specifically denote the note format or context that "annotation" does. Marginalia refers to notes written in the margins of a text, but it encompasses a broader range of comments and thoughts and is not restricted to the critical or explanatory focus that "annotation" specifies. Therefore, "annotation" is the most fitting term for critical or explanatory notes accompanying a literary work.