A nofollow link is an HTML attribute that you can add to a link to tell search engines not to follow the link. This is useful if you don’t want your links to count towards the ranking of a page, for example if you’re linking to a spammy site or if you’re paid to promote a site.
nofollow links are contrasted by dofollow links, which do the opposite!
The nofollow attribute should also be used on comments to prevent user generated content from getting free backlinks.