This graphic shows four levels of a hierarchical tree.
Level 1 contains the root/parent node of the tree. A root node is always at the top of a hierarchical tree. The root node has two "children" at Level 2. Each of these children in turn has children at Level 3. Therefore, the Level 2 nodes are parent/child nodes.
At Level 3, some of the nodes in turn have children at Level 4. These Level 3 nodes are parent/child nodes. Other Level 3 nodes have no children at Level 4. These Level 3 nodes are child/leaf nodes. A leaf node has no children at lower levels.