If you've been around this writing thing for very long, surely you've heard of the "story arc." People talk about character arcs, and plot arcs, and how one should build and sustain them. I talk about them myself, because I can feel them in my work as I write - but I know of a number of people who have difficulty with the idea of the arc, in part because they say they don't know what an arc is and how to detect one.
I have a very simple definition for you. A story arc is created when something in your writing gets repeated three times. In geometry, you use three points to plot (and confirm) a line; in a story, three points create an arc. In geometry, if you only plot two points, you can't be certain you haven't made an error, so the third point confirms that you've plotted the line correctly. In a story, if you've put in two points, then people will sense an arc forming and be looking for a third point.
Here's an example. I have an unnamed character appear in a concert scene chapter 1 of my book; the protagonist notices him as a person out of place. I then have this same character appear at a concert in chapter 3; the protagonist isn't sure this is the same guy, but approaches and speaks to him. Then in chapter 10 he goes to another concert. This is "strange guy appears at concerts" arc, and I expect that when my protagonist gets to the concert, he'll be unconsciously looking for this guy - as will my readers. He won't be there, but he'll show up in another place. This very minor character therefore has an arc in my story. His first appearance sets him up, his second appearance (and the fact that more happens in it) establishes that readers should be looking for an arc, and that way when you want to put in the next piece, you know people will be looking for it.
That's a very minor arc. A major arc can have many more points contributing to it. Each of my characters in For Love, For Power has two major arcs: a love arc, and a power arc. It makes sense! What this means to my writing is that in every chapter, I need to make sure I put in something to advance the point of view character's love arc, and his power arc. If I don't contribute to each in some way, then readers will have the sense that I'm dropping balls and I am not helping them to keep track of things properly. However, because these are large-scale thematic arcs, I can contribute to them in all kinds of different ways. Tagret's love arc can involve his friends, or his mother, or his girlfriend - and each of those can be considered as its own arc, but because they all contribute to the larger thematic arc, I can take turns developing them. Tagret's power arc can involve his father, or his brother, or other politicians.
So to define arcs, we need three points of repetition. This repetition is flexible. It can be a single object. It can be a single character. Or it can be a type of behavior on the part of a character (such as risk-taking). It can be a kind of thought or worry that the character has. But so long as we can be detected as returning to whatever this thing is, we can be construed as creating an arc. Arcs can also have subordinate arcs inside them, such as when instances of Tagret seeing his girlfriend contribute to his larger thematic love-arc.
You might be asking, "Do readers really know we're doing this?"
Well, they may not know it consciously, but humans are very good at detecting patterns of repetition on a subconscious level. It's one of our language-learning skills, and we do it all the time without thinking. In fact, as writers we are usually creating arcs subconsciously rather than deliberately. However, it helps for us to put a label on them and deal with them consciously, because if we do this, it's much easier to keep them under control.
Take a look at your draft. Try to identify the repetitions. Similar situations, or characters, or behaviors, or imagery. That's where you're creating your arcs. Here are some things to look for.
1. Am I only putting in two points of the arc? If so, chances are your readers are looking for a third, and will miss it if it's not there. Either complete the arc, or break it by removing the second point.
2. Am I precisely duplicating the first point with my second point? If so, the repetition may feel strange or uncanny. In an arc, usually the second point develops on the first (if only slightly).
3. Can I put a name on the larger thematic arcs? If you can, it will really help you move your outline and your book forward. If you can't, you might be wandering off the core point of your story.
4. Am I regularly contributing to the larger arcs? There are a lot of ways of doing this, but if you can identify the larger arcs, it will become easier to contribute to them and keep all those balls in the air.
5. Do I have a lot of similar small arcs? If you're struggling with the question of larger arcs, see if you can find patterns in the smaller arcs that you're using. That can help you detect larger patterns and get a better orientation on the overall trajectory of your project.
I hope I've given you some ideas that will help you articulate with the issue of the story arc. Arcs are a very important conceptual tool, and extremely useful, so good luck with them!