A narrative thread, or plot thread (or, more ambiguously, a storyline), refers to particular elements and techniques of writing to center the story in the action or experience of characters rather than to relate a matter in a dry "all-knowing" sort of narration.
In this sense, each narrative thread is the narrative portion of a work that pertains to the world view of the participating characters cognizant of their piece of the whole, and they may be the villains, the protagonists, a supporting character, or a relatively disinterested official utilized by the author, each thread of which is woven together by the writer to create a work.
[1] A classic structure of narrative thread often used in both fiction and non-fiction writing is the monomyth, or hero's journey, with a beginning, a middle, and an end.
First, typically the harmony of daily life is broken by a particularly dramatic event that leads into the main story.
Then, second, the plot builds to a point of no return, from where the protagonist – who need not be a person but may be an organization or a community – has no choice but to deal with matters, and thus is tested.