The drawing objects of Graphic Tools add a mechanism to creates a hierarchy of drawing, when a "drawing" is (typically) a simple Graphics Ports drawing operation. The hierarchy specifies the geometry of each node in the hierarchy, so the whole group of drawings can be manipulated as a single object.
The lower level interface allows you to create drawing objects and manipulate them. The higher level interface allows you to generate graphs of functions or bar charts, where "generate" means create a hierarchy of drawing objects. The higher level functions are useful on their own, but they also give examples of how to create high-level objects from drawing objects. You can look at their output to get a better idea how to write your own Graphic Tools code.
The Graphic Tools interface is defined in the package LW-GT. To use it, you need to load the "graphic-tools" module:
(require "graphics-tools")
CAPI User Guide and Reference Manual (Unix version) - 25 Feb 2015