“A hierarchy (from the Greek hierarkhia, “rule of a high priest”, from hierarkhes, “president of sacred rites”) is an arrangement of items (objects, names, values, categories, etc.) in which the items are represented as being “above”, “below”, or “at the same level as” one another. Hierarchy is an important concept in a wide variety of fields, such as philosophy, mathematics, computer science, organizational theory, systems theory, and the social sciences (especially political philosophy).” - wikepedia
It is also present in 3D applications to help organise / structure your 3D scenes, and can play an important role in figuring out your 3DSpaces/ creating setups in your rigs.
Here is a look at a hierarchy in a 3d App;
Besides organising your scene, it also defines a parent/child relationship.
The only direct links in a hierarchy, insofar as they are hierarchical, are to one’s immediate superior or to one of one’s subordinates
In the image above if you move the head, the child eyes will follow. If we move the arm, hand will follow.
Sometimes we refer to a 3dHierarchy as a chain.