Attribute rel = »nofollow» It is a micro-tag in the HTML code of a website. It is used to tag hyperlinks so it is not taken into account in the Google index. Thus it tells the search engine bots that they are not allowed to follow the links while crawling or crawling the site.
Google introduced the rel = nofollow- attribute in 2005 and other search engines adopted it. Google asks website operators to use nofollow links to non-editorial links. In this way, the links in the guestbook entries should not influence the development of the index. Many link builders use these links to manipulate the backlink profile of a website. For Google, backlinks are a very important criterion for the development of the index, which is why they use these links in forums, guestbooks and other records. This led to misuse of startup spam to change the profile or backlink pyramid. Google's reaction was the rel = »nofollow» attribute which should reduce spam. 
Currently nofollow links have several apps. Two are considered legitimate by Google:
1). First of all, paid links should not influence the algorithm to appear in search results, since Google also value websites for their referral links. Attribute rel = »nofollow» it is a referral, but the linkjuice is not broadcast. Google recommends this so that users are not fooled by paid links. Tagging paid links with the nofollow attribute - is comparable to flagging an ad.
two). Second, mark content as untrustworthy when you intend to link a site, but distance it from a bad neighborhood or bad neighborhood. This is always the case when website operators are unwilling or unable to accept responsibility for the content of links. If a user writes a comment and links a web portal, it's your own responsibility. The webmaster has been spared because he did not tag the linkjuice. Webmasters distance themselves from the target of the link and ask Google to treat the website and the target of the link separately.
3). Third, the use of nofollow attribute- for internal links it should be excluded by the crawler. It remains a myth that internal links are part of the architecture of the web portal. Many webmasters usenofollow to link to print and other insignificant content such as print templates and forms. But this does not have any advantage, in fact it is the opposite, because the valuable linkjuice is neglected and according to Google it interferes with the natural flow of the pagerank. An easy-to-use information architecture shouldn't be interrupted by an attribute rel = nofollow, because the user is in the foreground and not in the crawler. To keep the site out of Google's index, it is wiser to use a noindex meta tag and a lock via robots.txt.
The attribute (also: nofollow -Tag) is used as a link duration in HTML code:
All links on an HTML site must be tagged with "nofollow", it is also workable in the header:
The latter is usually used on websites where it is feasible to leave comments, for example, forums, guestbooks and blogs. Therefore the comments are excluded by the crawler, but at the same time trusted users and their links are also excluded, which in return might not be necessary. Therefore an ideal control of the links and the transmission of linkjuice would be the first option.
Importance for SEO
Attribute rel = nofollow Caution required: It should be used as recommended by Google. This is because the linkjuice flow can be specifically changed, which might appear abnormal under certain circumstances. As the rel = »nofollow» attribute is widespread, each site also has devalued links and is part of a natural (external) link profile. Even though this assumption is justified, it should only be used at specific points: with paid links or to avoid spam comments. The attribute must not be misused to achieve such a profile. This could disrupt the linkjuice. Using nofollow for link building requires caution.