nofollow is a flag that is used in the MediaWiki software to control the behavior of external links. Wikipedia uses it to prevent search engines from following external links and giving them credit in the search rankings. This is ostensibly to prevent spam from being added to Wikipedia.
Nofollow can have a distorting effect on search engine rankings. If Wikipedia takes content from another website, reworks it, and gives credit through an external link, Google will take this situation and put Wikipedia at the top of the search results, even though the author of the external site created the content and was cited in Wikipedia.