Stub pages are short articles that are in need of more detail than they currently attain. If a conctributor comes across a stub'd page then he or she are invited to add information to that page if they are knowledgeable in that certain field.
If a contributor comes across a page that is under 750 Bytes (You can do this by looking at the pages history or in Special:ShortPages) then a user could flag the page as a Stub, or just add more information. To do this you must add to the page via he edit feature.
Once a page is flagged as a stub then they will automatically be categorised under Category:Stubs so contributors can browse at all marked pages with ease.