A no follow link is a link that does not boost PageRank, does not count as a point in the page’s favor, and doesn’t help a page’s placement in the SERPs.
In SEO, no-follow links are the links which are not allowed to get indexed by search engines, therefore they are not good for SEO. They are used to generate website traffic.
No follow is a link attribute that instructs spiders not to crawl the particular backlink on the webpage. Hence these links are never indexed in search engines. They are only used to build traffic from various external webpages.