In the eyes of the major search engines, a link to a webpage is considered a "vote" for the quality of the linked-to webpage. The more high-quality links a webpage has pointing at it, the higher the value the search engines will typically put on that page.
Sometimes you might want to link to a webpage that you don't want to vouch for. If you want to link to a webpage without bestowing search engine benefit on it, you can use the rel="nofollow" attribute like this:
<a href="http://example.com/" rel="nofollow">link</a>
"Nofollowing" a link is a way to tell search engines that you don't vouch for the webpage that you're linking to. Search engines won't follow the link, and your site won't be helping the linked-to site rank higher.
