You may often find that you are only interested in certain nodes of a graph. Other nodes may be of no interest and it is useful, especially in large graphs, to be able to remove their children from the display.
Notice that some nodes have a small circle drawn alongside them, as shown in Expanded and expandable nodes. The circle indicates that the node is not a leaf node, that is, it has children. Moreover, the circle is filled black if the node is currently expanded, and is unfilled if the node is currently expandable (also referred to as collapsed).