Whenever you select any of the elements created by a drawing tool (pen, pencil, marker, etc.), you have the option available to "Allow Line Editing". If you enable this option you will have the ability to edit the stroke by manipulating the points that make up the stroke as well as the control points that control the strength, and direction of the curve.

When you first select a stroke and enable line editing, you will see a series of small squares running along the line, connected by a series of lines. These are the points that make up the stroke. You can click and drag any of these points to reposition them. You can also right click on one if you wish to delete it.

The strokes you draw are rendered as bezier curves, which gives you the opportunity to not only edit the points that make up the stroke, but also the ability to control the amount and direction of the curve between 2 points on the stroke. Whenever you select one of the points on the line (one of the squares), it will be highlighted, and 2 circular points will appear connected to it. These are the bezier control points. Drag them around to see how they can control the curve.