"Nofollow" provides a way for webmasters to tell search engines "Don't follow links on this page" or "Don't follow this specific link."
Originally, the nofollow attribute appeared in the page-level meta tag, and instructed search engines not to follow (i.e., crawl) any outgoing links on the page. For example:
<meta name="robots" content="nofollow" />
Before nofollow was used on individual links, preventing robots from following individual links on a page required a great deal of effort (for example, redirecting the link to a URL blocked in robots.txt). That's why the nofollow attribute value of the rel attribute was created. This gives webmasters more granular control: instead of telling search engines and bots not to follow any links on the page, it lets you easily instruct robots not to crawl a specific link. For example:
<a href="signin.php" rel="nofollow">sign in</a>