Hierarchy is used in all arts: music, illustration, composition, writing, architecture and even cartoons. At least well done thoughtful ones.
It means basically that there is a planned order of structures. Some structures are more dominant than others. The largest structures have distinct forms and are in turn divided into lesser forms - that have to conform to the laws imposed by the larger structures.
Take that cactus for instance. It has an overall shape, wide and fat and tapering towards the top.
It's then subdivided into rows of segments that fit vertically within the overall shape.
It's again divided horizontally by a a lasso.
Then each half segment of cactus has little quills sticking out- following thew perspective and position in space of the individual segments.
This principle of hierarchy, I believe, is your greatest artistic tool. It will help you understand how to put together important clear statements of either your own invention, or to follow through on someone else's. Without the use and understanding of hierarchy, you are left with anarchy and praying for lucky accidents to make your work sensible.