Some lingo I've picked up recently is "plotting" vs. "pantsing". A plotter outlines plot points and character sketches from start to finish before the novel is written, while a pantser, for lack of a better phrase, "flies by the seat of their pants" and constructs the novel as it comes, changing plot ideas, etc. as the writing happens.
I'm an admitted pantser. I prefer my characters tell me what they want to say (instead of me making them speak), and often times the plot will go in unexpected directions as I'm writing everything out. But the fall-out results in a bunch of plot threads that aren't connected, and a narrative that becomes disjointed.
When I wrote my first novel, an event led to a plot twist, which led to three more sub-plots and...you get the idea.
A former writing instructor offered the following advice: Find the 2-3 most important story threads and either discard the rest or save them for later books. My discarded plots are now acting as working outlines for the second and third books in my series (and I even know what happens at the end of each book).
So how do you know which threads are important and which aren't? Above instructor also encouraged me to outline everything in an excel chart--with chapters across the top and the characters down the side. Something like this: