The “nofollow” tag lets you not follow the links that are associated to them to your site’s rankings. It is great for SEO as you are able to filter out what sites you should follow or not thus preventing spam links in your blog. However, you don’t need to worry about it too much because Google algorithms change every day and the old methods of fooling search engines are long buried in the deep, especially after the Google Panda update.
It is still a debatable issue to whether a nofollow attribute can do much benefit in SEO or not, but if you still want to add nofollow tag to external links, here is how to do it by just using a simple plugin.
Read the rest of the tutorial here: http://www.wpexplorer.com/nofollow-external-links-wordpress/