Subpages are pages separated with a "/" (a forward slash) from their 'parent' page.
These are often used to create additional pages in "user space", when the pages are the work of a particular user and not for high-profile display, e.g. for drafts, to-do lists and the like. If your user name is Jane, then your userpage has the title "User:Jane" and you might create a subpage called "User:Jane/todo".
See also
- {{list subpages}} template to display all the subpages for a specific page.