Allows you to display a list of the child pages of the currently viewed page.
Subpage Listing allows you to take full advantage of WordPress Pages' hierarchy by generating a navigational tree of the pages below them (subpages).
For example, say you have a parent page called "Parent." Now, say you have 3 pages under "Parent," called "Child1," "Child2," and "Child3." Now, say that you have 2 pages under "Child2" called "Grandchild1" and "Grandchild2."
Subpage Listing would create a navigation tree for "Parent" that looks like this:
This allows you to very easily create a complex hierarchical structure that can be browsed.