This template should be used to mark an article or section as not being a complete description of what it describes. A description of the missing subject matter should first be placed in the associated discussion page, so that someone who wants to fix it can know what needs fixing.
You should not use this willy-nilly if you see something on the topic that isn't there. You should instead fix the article yourself. This template is only for articles that contain lots of missing information, such that personally fixing it would take some time.
It should also not be used if the information already exists in another article, unless that information is more appropriate in this article. Generally, you should simply provide a link, using Template:Main where applicable, to the other page.