A tag is simply a key word that links your hub to other hubs and questions. Simply write in the tag box a word that will help to identify your hub.
For instance: I wrote a hub on animal shelters and the pet population. Tags I included are--animals, pets, population, euthanasia, ect. That is what a tag is for. The more you add to your hub, the better the circulation your hub will get. Just make sure the tags are accurate.