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:
User-agent: *
Disallow: /wp-admin
Disallow: /wp-includes
Disallow: /wp-includes/js
Disallow: /trackback
Disallow: /*~*
Disallow: /*~
Disallow: /cgi-bin/
Disallow: /wp-content/plugins/
Disallow: /wp-content/cache/
Disallow: /wp-content/themes/
Disallow: /feed/
Disallow: /comments/
Disallow: */trackback/
Disallow: */feed/
Disallow: */comments/
Allow: /app/uploads/
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.