Q: What are citations?
A: Citations are references to printed comic books that are linked in-line in the text of an article (see the list of citations for the Iron Man article here: http://marvel.com/universe/Iron_Man_%28Anthony_Stark%29#cite_note-0. Citations help enhance your article by providing references to source material without overloading the article with lots of extraneous text.
Citations also help prevent questions of accuracy (and the pointless catty arguments that result from those questions) from arising and will help improve your chances of getting articles approved.
Q: What's the difference between citations and the "significant issues" box?
A: Significant issues are a general list of important issues for a character. They exist so that readers of the bio can get a broad overview of books to read about a character. Citations are used to support specific statements within a bio. There may be overlap between books in citations and books listed in the significant issues section, and that's ok.
Q: Where should I add citations?
A: You should put citations in the main text of your article (not in the significant issues, or bibliographical boxes), next to the incident or event which occurred in the issue cited.
Citations work best when they reference something that happened in a single comic, not something that happened over a range of comics or over a long period of time.
For example: "Over the years, Spider-Man fought many enemies." <- not a good place for a citation "A burglar killed Peter Parker's uncle Ben." <- a good place to cite Amazing Fantasy #15
Q: How do I add citations?
<ref>[series title] ([series start year]) #[issue number]</ref>
<ref>Amazing Spider-Man (1999) #600</ref>
If everything goes well, you'll see a link to your citation in the text of the article that links to a list of cited issues below the article. If the issue you cited is part of Marvel Digital Comics Unlimited, you'll see a "read online" link after the issue title.