A hierarchy can be defined as:

Most import to less important
Big to small (Figure 1) Hierarchy By Words And Graphics, below)

The Important of hierarchy

Hierarchy is used to communicate importance to the user
Humans process information visually long before they read it
The human brain group objects by shape, colour and size
This means that users look for hierarchy clues to tell them what is important or not
This hierarchy by size is often referred to as ‘visual relationships.’
Remember that the more text there is in each chunk the less likely the user will read it