Dofollow and nofollow are link attributes. "Nofollow" provides a way for webmasters to tell search engines "Don’t follow links on this page" or "Don’t follow this specific link."
The No-follow value instructs search engines that a hyperlink should not influence the ranking of the link’s target in the search engine’s index. So a search engine will not consider the link important. Nofollow links won’t drive much or any link juice to your website.
Dofollow links perform in a completely opposite manner: they grow your domain authority and serve as a positive ranking factor.
These attributes may be found on three levels of links:
Link level is the most local, it tells the search engine not to count a single link on the page: link text
Page level means that this part of the code will tell a crawler/search engine not to follow links on the full page. You can exclude links from crawling with the meta tag:
Site level involves robot.txt which indicates if crawlers or search engines can or cannot crawl parts of a website. A Nofollow instruction is specified by "Disallow" tag. The basic format looks like:
User-agent: [user-agent name]
Disallow: [URL string not to be crawled]