The ACTION in a work of fiction is that series of events that constitutes the plot, or what the characters say, do, think or in some cases fail to do. Orderly ACTION differs from aimless or episodic activity; an ACTION customarily has a beginning, middle and end.
So it's not just the entire work that needs a beginning, middle and end, it is the ACTION as well. Or what one might think of as "scenes." And each scene must mean something. There must be a purpose for its existence in the manuscript-- to advance the plot, to explore the characterizations, to set the tone or develop themes. "But this really happened to my uncle Ned" isn't a good enough reason to include meaningless ACTION into your story.
(This post has been inspired by and in some instances, directly quoted from A Handbook to Literature, 8th Edition, by William Harmon and C. Hugh Holman)