The designator source refers to the note from which the exported links will arise. In most (all?) cases this equates to both this and current, so the object 'source' makes it easier for the user to specify generically from where data is drawn.
See also - destination.
^basicLinks("<ul>","<li>^value($Name(source))^ - > ","</li>","</ul>")^
will export a list of links, each destination note's Name preceded by the source note's name and a -> pointing to the destination note's name.
Note that the designator can be used in the link export code's item prefix and/or suffix arguments but this designator cannot be used with ^value^ and attribute references. Meanwhile, the link's anchor text always remains the Name (i.e. title) of the link's destination note, linked to that note's page's URL.