A stub is an article that is too short, but not so short as to be useless. A long article on a complicated topic may be a stub; conversely, a short article on a topic of narrow scope may not be a stub.
Another way to define a stub is an article so incomplete that an editor who knows little or nothing about the topic could improve its content after a superficial Internet search or a few minutes in a reference library. An article that can be improved by only a rather knowledgeable editor, or after significant research, may not be a stub.
Sizeable articles which lack wikification or copy editing are generally not considered stubs.
Once a stub has been properly expanded and becomes an article rather than just a stub, you or any editor may remove the stub template from it. No admin action or formal permission is needed.
Stubs have their bad sides. Readers can get confused by a too-short article and stubs can give a bad first impression if people haven't seen other wiki articles.
Mostly, though, stubs are a good thing. A stub is the seedling from which the full plant of an article emerges. You can add a stub, and other users will come along and add more information to it. Someone else comes in and reformats the article, someone else adds photos, etc, etc, etc... eventually, the tiny one-sentence stub becomes a healthy, useful article.