Hierarchy is when an element appears more important in comparison to other elements.
When something is larger than other elements, it appears more important. An example are headlines on newspapers. Another exmaple is the Pocket app.
An element can also seem more important if it is an irregular shape. An example is the circular shape of the profile picture in Instagram versus the square shapes of images.
Elements can be placed higher than other items, which causes focus to go to the top. Circles at the top of apps are a good example.