Here we have a simple 'sky' background with a plane which we've drawn with the Pen Tool and a white 'trail' drawn with the Line Tool.
So far, so simple. But by selecting the Width Tool from the toolbar:
we can begin to manipulate this white line. The Width Tool can be used either manually - by clicking anywhere within the line and dragging towards or away from the axis of the line, or, by double-clicking the line axis, entering coordinates in the Width Point Edit dialogue box.