Subpages are groups of pages that share a common prefix, which itself is another page. While this is also possible with "classic" wiki, by using names like SomeTopicSubTopic, the use of SomeTopic/SubTopic allows better navigational support, and you can omit the common prefix when linking from the parent page to the child page.
Thus, by using "/" to concatenate several WikiNames, you can create arbitrarily deep hierarchies (within limits, especially the length of filenames on your system). In reality, subpages are normal pages that contain a "/" in their name, and thus they are stored besides all other pages in the file system. Subpages are a configurable feature, but they're on by default and it's recommended that you keep it that way, since the help pages themselves use this feature.
Links to sibling pages start with "../", for example ../SubPages (a link to this page).
* HelpOnEditing/SubPages * [[HelpOnEditing/SubPages|This very page]] * /ThirdLevel * [[/ThirdLevel|A page below this one]] * [[/sub page with arbitrary page name]] * [[../SubPages]] * [[../]] (anonymous parent link) * [[../|free parent link]]
../ (anonymous parent link)
Please do all of us a favour and don't create the /ThirdLevel pages, it's just an example!