What is :source in the Token module? I look at the List of tokens and I see a lot of tokens with the same name and description, but some node have :source appended to it. Some of these, such as [node:title], I can understand, since title is a field of the node. But what is [node:source:title], or [node:source:source:title]? There's even an [node:source:source:source:source], which has the description 'The source node for this current node's translation set.'

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to different languages, so if the source post is significantly updated, the editor can decide to mark all translations outdated.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)