Stubs are meta callouts pointing out that a page is incomplete. Stubs are considered too short to provide complete coverage of their topic, but should strive to at least outline the core ideas and concepts. Stubs should clearly outline their topic in at least a short one-paragraph description.
The existence of stubs ensures the proper linking of articles across the encyclopedia, maintaining the flow and structure of page links. Without stubs it can become difficult to track when a concept is mentioned and retroactively link past mentions. Stubs subvert this by allowing complete pages to link to unfinished concepts which can be returned to at a later date.
Readers can contribute to stubs by leaving comments. Contribution by recommending further development, suggesting additions or revisions, or proposing merges or splits of the articles topics are all appreciated.