Have you ever read a story that didn't seem to have a lot going on? One that was all action and no subtext? One that focused solely on what was physically happening and not enough on what was going on underneath? Stories such as these suffer from a severe lack of narrative cohesion between A plot and B plot.
A plot can be described as what is actually happening to the characters, what they say and do that drive the narrative forward. This part is relatively easy, and most writers seem to get their point across well enough early in the writing process. The problems then come in, however, when the writer begins to need to justify certain actions to the reader, as the characters motivations can sometimes be difficult to express without outright stating them. All the same, it is imperative that the writer understand the importance of character motivation, as it ties neatly into the definition of B plot. B plot can be defined as what goes on in the background, underneath what we see the characters say and do. This is the true meat of the story, as it provides context and meaning for the A plot, or action that takes place.
For example, let's pretend I have written a story about a man who shoots another man. Is that interesting? Well, it might be interesting in and of itself. For about five seconds. Until, that is, you as the reader start to question why the man shot the other man.
"Oh, I don't know, he just did." I tell you.
Understandably, you would probably be pretty upset at me for writing such utter nonesense. Without some sort of narrative backing to prop up the action I have written, the scene is meaningless. So a guy shot another guy. Whoop-dee-heckin-doo.
Now, let's pretend I wrote a new story, improving on the last. This time it's about John Waycko, an exconvict who's wife and her secret lover framed him for a murder in order to get him out of the way. Now John has escaped prison and is back for the blood of the man who stole his wife and put her up to the lie. When his wife is out at the mall, John corners the creep in his bedroom, and blasts his face off.
Isn't that more interesting? Perhaps it's not your personal cup of tea, but it's definitely someone's! Quite frankly anything would be better than the first story. Now we have context for why John shot the other man. And if we were to take it further, we could add in the context of his wife's feelings toward him, why she did it in the first place, and perhaps what the wife-stealing man's motivation was for attempting to ruin John's marriage and life. Almost any answer to those questions would be better than, "Because he wanted to." That would be boring and the reader would be completely justified in burning your book on the spot to save anyone else from the tragedy of having to read it ever again.