The term describes the way of content arrangement, as well as relations between design elements. Dealing with the interface, the user notices visual hierarchy, understanding that the first element taken the attention is the most important one. The following elements have subordinate relations with each other.
Here are some basic principles of building up the hierarchy in UI:
The size The huger element gets the bigger attention
The color Brighter objects are likely to be first seen
Contrast Two elements of dramatic contrast will catch the eye faster
Alignment The high-priority of data is always closer to the top
Repetition Using same style for repeated elements lets the user determine relative elements at a glance
Proximity Less is the space between the elements, higher is the relatedness between them