HTML attribute developed by Google that, added to the URL of an outgoing link on a page, allows it to be automatically ignored by Google's webcrawler ( Googlebot ) when indexing the web. The tagged links do not affect the PageRank of the web, which allows to prevent, for example, that blogs and forums are affected by the spam that they can host in their comments.
The nofollow tag is used in web positioning so that the source page does not transfer link juice to the recipient of the link.