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.
Links to sibling pages start with "
../", for example
../SubPages (a link to this page).