From Google, "Nofollow" provides a way for you to tell search engines to DONT FOLLOW the links on this page...or to not follow a specific link. The value: tell Search Engines to NOT follow specific links on a page. Ex: dont follow links in any comments. This is because spammers use the WordPress comment areas to post links to their sites. Neither search engines nor the site owner want that to happen.
Here's some more information about "Nofollow"
http://support.google.com/webmasters/bin/answer.py?hl=en&answer=96569
http://weblogs.about.com/od/bloggingfaqs/f/FAQNoFollow.htm