An HTML link attribute that communicates to web crawlers and search engines that the link to the destination web page should NOT transfer SEO equity (ie it shouldn’t give SEO benefit to the recipient).
It's a directive used to tell search engines not to follow a specific link or the links on a webpage or count the link towards its PageRank.
According to Google’s guidelines, any link that is unnatural (like you paid for a press release, or you gave a journalist a perk for writing about your product) should have a nofollow tag.
When a link from one site does not pass SEO credit to another. Do not use nofollow when linking to internal pages in your website. Use it when linking to external pages that you don’t want to endorse.