This button lets you look at your organization's view either in Hierarchical or Tree view. To get to this button simply click on the Admin on the side panel and it will bring you to the organization section.
A hierarchy is an arrangement of nodes in which the items are represented as being "above", "below", or "at the same level as" one another.
A tree view or an outline view is a graphical control element. To expand the tree simply click on the little arrow next to the nodes name.