The following describes what darts are.
A piece of fabric is a flat 2D object. The human form is a 3D object. Wrapping a single piece of fabric around the body often results in folds. A fold, in essense, is excess fabric. To eliminate unwanted folds (without stretching the fabric), simply gather all nearby folds together into one large fold and cut that fold away. If the fold´s fullness is at the edge of the fabric piece, the resulting cut will most likely look like a V-shaped notch into the outline of the fabric (see (1)). That´s a "basic" dart, or a dart with one endpoint. If the fold´s fullness is in the middle of the fabric piece, the resulting cut will most likely look like a diamond-shaped hole in the middle of the fabric (see (3)). That´s a "double" dart, or a dart with two endpoints. (Confusingly, MD does not have a name for the first type of dart, and calls "dart" the second type of dart). Furthermore, as you´ll see below, darts are closely related to seams. You can think of darts as incomplete seams, or you can think of seams as darts that have been slashed through their endpoints. The bottom line is that darts and seams are the two basic methods for dealing with unwanted folds without stretching the fabric.
So, next time you try to wrap fabric around the body and you get unwanted folds, use a dart (or a seam)!