What is :source in the Token module?
:source you can understand as parent of that node when you get in touch with drupal translation you can easily understand what hierarchy this :source is referring to.  

as per above link:

Translations are managed in sets of posts, which represent the same information in different languages. Only content types for which the administrator has explicitly enabled translations could have translations associated. Translations are managed in sets with exactly one source post per set. The source post is used to translate to different languages, so if the source post is significantly updated, the editor can decide to mark all translations outdated.

so :source here is referring to the parent node from which it gets translated (if there are translation enabled if not then you need not to worry about :source)