A drawing is something drawn on the screen using draw_xxx() functions. This can be a circle, square, line or whatever they can draw. A drawing is associated with a certain DrawID, returned by the function which drew the drawing.
A drawing also has a certain z value, initialized when drawn and at that point equal to the value set using drawing_z(). The drawing will behave like a process with the same z. When drawing_map() is used, that z value is ignored and the drawing is just drawn on the specified graphic.