Yes, Robots.txt file is essential for the SEO and it is the text file, not HTML file. According to Google Guidelines, a robots.txt file gives instructions to web robots about the pages the website owner doesn’t wish to be ‘crawled’. It contains the following code:
sitemap: http://domainname.com /sitemap.xml
The "User-agent: *" means this section applies to all robots. The "Disallow: /" tells the robot that it should not visit any pages on the site.