A stub article is any article in Wikitravel that has very little information and is of no use to a traveller in its current state.
The question of which articles are stubs is a tricky one. Very short articles are stubs - if an article is only 2-3 sentences long, it is almost certainly a stub. But just increasing the length is not enough to make the article lose its stubbiness. It is better to look at its usefulness to the traveller. A three-paragraph article that just talks of the ancient history of a destination is much less useful than a couple of sentences each about how to get in, what to see and where to sleep.
If an article is worth writing about on Wikitravel, it will usually fit into one of the Templates. If a template is inserted and one or two of its sections are filled out, then it is probably time to move it to outline status.
Pros and cons of stubs
Stubs have their bad sides. Readers can get confused by a too-short article: Is that all there is to say about the article? Is that the expected length of articles for Wikitravel? Where's all the info? Stubs can give a bad first impression if people haven't seen other Wikitravel articles. (But see below for a way to make stubs less confusing.)
Mostly, though, stubs are a good thing. A stub is the seedling from which the full plant of an article emerges. One Wikitraveller can add a stub, and other Wikitravellers will come along and add more information to it. Someone else comes in and reformats the article according to the Manual of style, and someone else adds photos. Eventually, the tiny one-sentence stub becomes a healthy, useful article.
If you set the Threshold for stub display value to something other than 0 in your preferences, links to stub articles will be shown in a different color than links to complete articles or to non-existent articles. The threshold value is a number of characters in the article; somewhere around 500 characters should give you a good idea of whether an article is long enough or not.
Note that this only shows short articles, and so it's a rough approximation of stubbiness. Some articles may need additional information, even if they stretch for hundreds of thousands of characters. We don't have the software yet to decide if an article covers its subject well.
If you make a stub article, or see one that someone else has made, it's good to add a little disclaimer that says that the article isn't done yet. It gives a bit of extra impetus to readers to add what they know to an article. There's special markup in our software to mark something as a stub. It looks like this:
...which makes this appear on the page:
You can add the stub message at the bottom of the page. This reassures readers that we know the article is not complete, and that it's not indicative of the overall quality expected out of Wikitravel articles. Also, it invites them to add whatever they can to make the article better.
Best of all, other Wikitravellers can use the What links here feature on the Wikitravel:Stubs needing attention page to check for new stubs needing attention -- although it'd be preferable if you did that when you first detected the stub.
It can help very much if you copy the appropriate article template to the stub article. For example, if the stub is for a region of a country, copy over the region article template. People are more comfortable if they can add just a little information at a time, rather than writing the whole article from scratch.
Taking out stub disclaimers
Alternatively, if a stub is about a topic that is never going to turn into a worthwhile Wikitravel article then it should be proposed for deletion if it doesn't align with our goals or else redirected to an appropriate article that the topic can be incorporated into. Redirection or deletion is often a pragmatic decision. There is little point in writing lots of tiny stub articles about small but nearby places (each with only one or two attractions) when a single article about the area all those places are in can cover all the attractions in one hit, and it will not be a stub, because the template can be filled out too.