A stub is a very short article that does not adequately cover the subject matter of its topic. A stub should be expanded so that it is no longer a stub. Stubs are listed in Category:Stubs.
What constitutes a stub is a judgment call. One rule of thumb is that an article that covers its subject reasonably even if superficially is not a stub. Another is that if someone who was generally knowledgeable but not an expert would need to do library research to add to the article it is not a stub. Another is that if the article has significantly more than 12 sentences and three paragraphs it is probably not a stub. But all of these are general rules of thumb, and may not apply in a particular case.