The best concept to explain hierarchy: “parent-child”. This parent-child model explains both the levels, and the one-to-many relationship.
As in a family you have one great grandmother, with many grandmothers below her. Each grandmother can produce many mothers. Every mother can produce children.
In this way you have an “upside down tree” structure, with the base growing thicker and thicker.
This post’s diagram explains hierarchies.
You can see how one parent can have many children.
You can see how information can be classified into levels.
For a specific working example on hierarchy, please check out geographical hierarchy.