Robots.txt is used to instruct bots what parts of your website, you want to show them and what not. Suppose you have a private folder that you do not want to be indexed then you can simply use: user-agent: * Disallow: /private
you can just control the stuff u dont want to make public or if u want to block any particular search engine
You are created a robots.txt file of a website.... that used a nofollow any webpages than add a that file in robots.txt