First, some context: this is not a universal method of looking at pattern draping/drafting. As far as I can tell, it's very 20th century, and mostly useful for western-style clothes and construction. I think of it as one way I can understand how to construct complicated shapes out of fabric, but definitely not the only one, and a lot of times it's sort of subconscious. So here goes.
Imagine you are trying to clothe a cylinder. It turns out that it is very easy to do; you just wrap a square around the cylinder, and you're done.
It gets harder when you want to clothe a sphere. You try to use that same square, and it wraps nicely around the center, but you have all this extra fabric at the top and bottom of the sphere.
That extra fabric is called too much fullness. You have to get rid of it, or control it in some way. One simple way to do so is with "gathers," which is when you cause the extra fabric to bunch up in soft wrinkles, like with a drawstring.
That's fine for a hippy or peasant look, but sometimes you want something a little more intentional looking, so you could instead make just a few measured folds instead. Those are called pleats (and of course there are many variations, like knife pleats and box pleats and so on*):
But you're still just controlling along the far edge of the fabric; perhaps you'd like a bit more definition along the entire too-full area? For that, we have darts. Darts are basically wedge-shaped chunks of fabric that are taken out by sewing their edges together. If you were to cut that chunk out instead, your fabric would flatten out to look like a map projection:
And if you wanted to match the surface of the sphere the whole way, you might just do a seam:
These methods exist on a continuum, of course: pleats are basically more careful gathers, darts are pleats that are sewn further, and seams are (sometimes) sets of darts that touch in the middle. They do have different implications for construction and the look of the final garment, though. For example, a seam can involve two different kinds of fabric, whereas a dart cannot. Gathers are the easiest to make (since usually you do actually just run a drawstring through the edge of the fabric), tend to be the "softest" looking, and work well with lighter fabrics.
Thinking of shaping fabric by "controlling fullness" is essentially a subtractive process -- you imagine how wide you're going to want your shape at its widest, then set out to reduce it elsewhere. Needless to say, this is not always how one wants to be thinking about a task. To my mind, the opposite of this way of thinking is the "slash and spread" method of drafting, which I might make a post about later. Would you be interested in reading it? (Other arguably related topics are fitting/design ease, and how to reposition darts.)
*I absolutely love this book by Colette Wolff, which is essentially a taxonomy of methods of shaping fabric. Many of them are good for controlling fullness.