The robots.txt file instructs search engine robots which web pages and sections they can or cannot crawl and index. This file is located in the root directory of the site (the topmost folder in the file system) and controls how search engine robots should behave when crawling and indexing the site. In addition to information about indexing the site, the robots.txt file contains the URL to the XML sitemap of the site.
Solidpixels generates robots.txt automatically for each site.