A subpage is a page that is stored "under" another page, and includes the parent page's name in its title, followed by "/" and the subpage's name. For example, User:Eloquence/Favorite Wikipedia quotes. When you visit a subpage, you will see a backlink (aka breadcrumb) near the top of the page back to the parent page.
Subpages can be useful for organizing and archiving project content, and for creating workspace under a user account. On a parent page, you can create a link to one of its subpages by typing [[/Name]]. If you end the link with a slash, e.g., [[/Name/]], the slashes are hidden in the output.
Articles don't have subpages. That is, subpages are not enabled in the article namespace, because many articles might qualify as subtopics of more than one topic. Instead, follow article naming conventions and include links to related subjects in the text.
For the subtopic structure of subjects, see Portal:Contents/Outlines.