A stub is Wikipedia's term for an article too short for its subject, and requires an expansion. It usually contains just a short definition of the subject, with no further details.
Whether an article is a stub or not depends on the subject of the article. For example, an article like Mr. Kurtz can contain very few details, and yet not be considered a stub - as there is no further information available on the character. However, if an article like Jerome Baldino would contain nothing but his name, relation to Twinsen and his residence, then it would definitely be considered a stub.
To improve the Twinsuniverse, one thing you can do is work on expanding the stubs in it. For a list of all the stub articles in the Twinsuniverse, see the Stubs category.
Note: When setting up the Twinsuniverse, lots of stub articles were created to get the articles listed.