The slug is a custom page path that permits to customize at will your page url (if you choose to).
With the slug value
the:slug:that:you-have-chosen
and a page url configuration of slug, the url of your page will be:
http://example/com/the/slug/that/you-have-chosen-pageid
where pageid is a short page id encoded
You can modify it via the metadata manager
To be used, the page url configuration should be on one of the available slug configurations