Which term refers to the writer's personal reaction, opinion, and insight?

Commentary

Explanation:

Gnerally, when an author halts the flow of a story to comment on something of importance, be it a theme or a motif, or a character, or even a situation that has come up in the story it is known as the segment of commentary.

In this segment the author is talking to the audience directly and not through a character or a story.

The writer is responding to where the story has till now taken the reader. Thus, the narrative is often contextualized from a commentary statement providing new insights and opinions.