Both the message text, and the message documentation are not helpful to understand what this is about.
This may need a bit of background about WLM. Wiki loves monuments is about uploading to commons images of monuments. For this goal, lists of monuments were done, from the list of countries heritage monuments. The images had to be uploaded including the monument id (given by the government). The link is to filter the list so that it only shows images which don't have an id present in the lists (a bad id). If you have ideas on a better description, feel free to add it.