If you are a requester:
Articles from time to time lose the wiki syntax they (hopefully) had when they were first created. When you notice such an article, add the "[[Category:Wikify]]" so we can go back and fix broken links, etc.
If you are an editor:
To wikify, go through the article tidying wiki-syntactical errors, broken links and adding missing links where appropriate. When your work on that article is complete remove the "[[Category:Wikify]]" tag from the article.