It should be used, much like other hatnotes, on articles which have generic names and content that differs from the general meaning of their titles. Current examples include Ridicule (an article about a movie, not about mockery) and Visage (an article about a band, not about faces).
If one parameter is used, the template will include a This article is about... description. If a second parameter is used, the template will use that as the word to reference. The word defaults to the name of the present article.