What is the narrative mode?

A narrative is a work which communicates a story, a series of connected events and details. For a work to be delivered in the narrative mode means that the characters, settings, and events are not merely recited as facts, but presented within the context of their interlocking relationships. In short, a story is told. The narrative mode of writing can be fiction or nonfiction, and told from a number of different points of view.
In the case of "A New England Nun," the story unfolds through a third person narration, or a person who is an entity not directly involved in the story. The writer chooses this form and point of view for several important reasons.
The crux of the short story is to elucidate Louisa's struggle for independence and a coherent sense of self in the context of an unwanted marriage. Louisa's identity is expressed and affirmed extensively through domestic chores and projects. The narrative mode lends itself to detailed descriptions which illuminate the actions and events of characters. In this case, the narrative style does well to communicate the daily minutiae of Louisa's life and give the reader an idea of how domesticity enriches Louisa's sense of purpose and self.
The narrative mode is also very much about weaving together many elements to form one cohesive whole. The narrative aptly captures Louisa's journey, as she too weaves together a life, an identity, from many discrete objects, feelings, tasks, and events.