In her story, "Happy Endings," Margaret Atwood explains to writers that there aren't many different plots--most of them end up the same way (people die). She shows through her different stories that beginnings are more interested than endings, and says after that, writers have to deal with the how and why. This means that once writers have the beginning and end of a story, they need to figure out the middle--the motivation, the details, dialogue, etc.
She also makes commentary for the readers and their expectations. Readers must parse meaning from these plots. In this story, Atwood explains the responsibilities of writers to create stories and how they must understand what readers expect. By giving multiple plot lines and then saying that there is the matter of the how and the why, it puts the responsibility on both the writer (to expand the plot) and the reader (to make meaning of the details the writer gives).