In part 1 of the assignment Bernie wrote:

• Add user annotation to links

It's just another model that would provide information about a link. It would have an association with a link and with a user, so in addition to having, say an annotation property, it would also have a link_id and and user_id to associate with these two tables. The assignment instructions say this should have been included as part of the code you were given, but it wasn't.