Repairing articles edit
Cleanup edit
In order to clearly and efficiently provide as much information as possible, Wikipedia has several editing guidelines to ensure that articles are kept neat and easy-to-read. The Manual of Style groups these guidelines into a type of user's guide to editing Wikipedia. It outlines how to properly structure articles, and how to take care of the little nuances of English grammar in certain situations (where and how to use punctuation, whether to use American, Australian, British, Canadian, or some other dialect of English, etc.). Other policies and guidelines refer to how to present that type of information. The Five Pillars of Wikipedia state that all information should be presented neutrally, and as such the Neutral Point of View, or NPOV policy is a very important policy to keep in mind when editing Wikipedia. Most pages on Wikipedia adhere to these policies and guidelines. For those that don't, we have cleanup.
WP:CLEANUP is a very large project with (currently) the third largest, and possibly oldest, backlog in the project, beaten out in size only by articles lacking sources and articles with unsourced statements. Pages get added to the cleanup category when an editor adds a cleanup template to the page. These templates point out to editors and casual readers that there is a problem within the page, and as such can't be relied upon to be completely accurate or even legible. Editors are constantly needed to work on these pages to resolve immediate problems to help improve things. This can be as simple as doing some basic copyediting for grammar or spelling, or re-wording sentences to remove bias, or in extreme cases completely re-writing entire articles.
While tedious, such cleanup patrols are essential to keeping Wikipedia running smoothly. The Manual of Style and WP:CLEANUP both give details on how to do all that.
Other edit
Once familiar with cleanup, as it is the most important and most needed job, there are several other forms of article repair you should know about.
Wikifying edit
Assuming that you've done the Wiki markup lesson, you've learned how to make an internal link, or "wikify" a word or phrase in an article. All you do is place two square brackets on each side of a word. (Four in all. Example: [[Purple]] would cause the text to display Purple.) Some articles lack or have large sections that lack wikified words, thus are tagged with {{Wikify}}.
NPOV (Neutral Point-of-View) violations edit
While violating NPOV is basically a sub-division of cleanup, it is a bit different than cleanup in the sense that it isn't necessarily written badly, it's just written biased. Really, all you need to do to make an article POV is to switch biased words into neutral ones and to remove opinions and rumors all together. Other than that, it's fairly simple.