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.
* HelpOnEditing/SubPages * [wiki:HelpOnEditing/SubPages This very page] * [wiki:Self:HelpOnEditing/SubPages This very page] * /ThirdLevel * [wiki:/ThirdLevel A page below this one]
Please do all of us a favour and don't create the /ThirdLevel pages!