Structure is the 3d equivalent of shape.  Here you can see some shapes and how linear contour lines can be used to wrap around the shapes to suggest structure using line only.  This is called linear structure.  We can also use value to suggest form and structure as well.  This is how we typically understand structure in the real world.  The way light falls on a structure reveals it’s form.  As things turn away from the light source they receive less light and thereby become darker.