Visual hierarchy is the order in which a user processes information on a page. By assigning different visual characteristics to sections of information, a designer can influence what users will perceive as being further up in the hierarchy. Below are ways in which a designer can influence perception:
When information design does not have a strong visual hierarchy, a user’s eye follows a predictable reading path. A designer has the opportunity to use visual hierarchy to reinforce these natural paths and draw the viewer’s attention to a focal point.
Source: Interaction Design Foundation