A stub is a very short article, generally of one paragraph or less, or possibly consisting solely of links to other articles or external web sites. A stub can also be a place to keep relevant information until a more well-rounded article can be written. A stub is a legitimate article on the subject at hand; it just needs to be expanded.
As with Wikipedia, a stub is an article which is clearly too short but not so short as to be useless. On a general manner, it must be long enough to at least define the article's title.
Unlike Wikipedia, many articles on HypertWiki begin life as directories or lists of links, and definitional text need only be inserted to help clarify the overall nature of the subject or as a guide to other articles available. When an article remains marked as a stub even though it appears to have substantial content, this is usually what is wanted -- some kind of overview of the subject, preferably with explanations of how the information available elsewhere in the article fits in with the big picture.
The nature of the missing text depends somewhat on the nature of the project; for any given project, see the project's Help page. If the project doesn't have a help page, write one or contact the wikimonster to ask about it!