See here: http://en.wikipedia.org/wiki/Help:Link#Subpage_feature

Essentially, prepending a wiki link with a forward slash should create a wiki link relative to the page the wiki link is on. Since Liquid doesn't particularly have subpages, it would be nice to implement this feature as a workaround to having to completely type out the full location of a wiki page.

For example, on the wiki page TestPageOne/AnotherTestPage/AdditionalTestPage, a link to /LastTestPage would link to /wiki/TestPageOne/AnotherTestPage/AdditionalTestPage/LastTestPage, instead of linking to /wiki/LastTestPage.