MediaWiki's subpage feature is turned off by default.
If the subpage feature is off (this is the default), then slashes (/) within a page name are simply part of the page name and do nothing special.
If the subpage feature is on (default is off and this is a per namespace setting), then slashes (/) within a page name break the page into parent and subpages, recursively, eg:
Link(s) back to parent(s) of the subpage(s) automatically appear at the top of each subpage. Note that these links do not appear however if the parent page has not yet been created.