The use of asterisks is an old print convention, used to signal a foot- or endnote in which the writer wanted to express extra information but not detract from the general direction of the prose. Other characters were also used, especially if there was need for more than one note. Later conventions include the use of a numbering system for extensive foot- and end-noting, particularly in some academic writing. Some, slightly more scholarly-oriented writing online maintains these conventions in essay writing and, of course, the use of notes for sources on Wikipedia is probably its most currently well-known use.
But there are other ways to express, codify, a desired intertextuality, especially when it comes to usefully citing and referring to sources, but also to generally create a richer textual environment for readers. (I hope I'm demonstrating that here, particularly for MIAD students who followed the double-asterisk linked from their course Moodle. Readers should follow the first asterisk on the Moodle page to the bottom of that section, like the bottom of a page; the second, double asterisks, brought you elsewhere on the internet.)