Nofollow is an HTML term of indication by which search engine spyder/crawler can’t pass the juice of the site to another site which is linked to it. It is the opposite of Dofollow Links. Where in Dofollow link, a site passes its juice to another site, same but oppositely in a Nofollow link, a site does not pass the SEO juice to the site which is linked it with.
Look at this Picture
In a Practical Example, we have 2 sites Blog A (rakeshsardar.com) and Blog B (nowthisdigital.com). An I’m putting a hyperlink on the text nowthisdigital.com, and make it nofollow. Then when the search engine spider/crawler will come to crawl the link and when it will found a Nofollow declaration then crawler will stop crawling the linked page and will not provide the SEO juice.
If you look at this post page source (In chrome right click and click page source), then you may see on the hyperlink text I declare it Nofollow using Html tag ‘rel=”Nofollow”.