The dialog option allows to modify a line or an arrow which has been created by the command Draw Arrow/Line from the Graph Menu or with Ctrl-Alt-L. On can also open it with a double click on an arrow or a line, or by selecting an arrow or a line and selecting Properties... with the right button of the mouse.
The first tab allow to change the color, the line type and the line width. This last parameter is set in pixels.
The Arrow head tab is used to modify the shape of the head of the arrow. The length is se in pixels and the angle is in degrees.
The Geometry tab allows to specify the start and end point of the line/arrow. The values are in pixels, and are relative to the top-left corner of the layer which contains the line.