Websites can use the HTML tag rel="nofollow" to tell search engines not to give any weight to a link for search engine optimization purposes. Many sites, including many of the largest on the Internet, do this by default, making the tag of questionable value if you are a search engine crawler. Therefore, many search engines make their own decisions about whether or not to honor these tags, and they usually don't.
So, if you can place contextually relevant backlinks on websites that tag links with rel="nofollow", do so. Don't let the nofollow tag discourage you.
History of relational tags
about the relationship between the linking page and the linking page. Some of the most common and meaningful to marketers are rel="ugc," which indicates user-generated content, and rel="sponsored," which indicates sponsored content. There are currently over 100 link relationships recognized by the Internet Assigned Numbers Authority. Most are arcane and not used by search engines in a meaningful way for marketers.
Nofollow links were established in the first iteration of the link vp media email lists relationship attribute (aka “rel” tags) back in 2005, and were proposed by Google’s Matt Cutts and Blogger’s Jason Shellen. They were created so that webmasters could tell search engines what the purpose of a given link was. This was primarily done to combat link spam, which was ubiquitous at the time.
Search engines were much more primitive in the mid-2000s. For a long time, search engines treated relationship tags, including nofollow tags, as a law. A nofollow tag prevented the site or page being linked to from gaining any SEO benefit . Plus, many search engines wouldn’t even index the link!
The evolution of Nofollow
Over time, website moderation technologies and best practices improved to the point where website spam was much less of a problem. Today, it’s fairly rare to see spam on high-quality sites like Wikipedia or Reddit. Most forums are pretty self-managing, most blogs have effective anti-spam tools, and even when they fail, modern search engines are able to distinguish what’s spam and what’s not based on context and other factors. Rel tags, or at least nofollow tags, simply aren’t necessary.
Google's Danny Sullivan and Gary Illyes announced this in a blog post in September 2019: