It is a concept that is used in UX design, graphic design and photography, where elements are placed in a certain way to guide the viewers eye in a specific order. A viewer has to be able to find the elements that has the highest priority of importance.
Visual hierarchy in photography
In photography, visual hierarchy plays an important role, because it helps the user to understand what is happening in the image. It can be achieved by placing the main subject in the middle of the image and using visual patterns.