"Loner Zettel" are Zettel that don't link with any other note, i.e. they have no links and are not linked to.
Since the key idea of a Zettelkasten is making as many connections and associations between relevant ideas as possible, loner Zettel don't contribute much until they have other notes that link to them. Thus, it's usually a good idea to integrate them with the rest of notes as soon as possible.