A permanent link (or permalink) is a link to a specific version of a wiki page. It usually brings a version number, also called oldid.
Take a look in the sidebar left. There should be a container "Toolbox" with the link "Permanent link". Clicking on it will load up the permanent link page. It has a special URL, for example on the Main Page:
You can see that 3748563 is the version number (oldid).