The 'robots.txt' file is a text file that webmasters place in the root directory of their web server to communicate with web robots, such as web crawlers and spiders (like Googlebot or Bingbot). It is the primary mechanism for implementing the Robots Exclusion Protocol (REP). This file does not technically restrict access to the content; rather, it serves as a set of guidelines that well-behaved bots should follow. It specifies which parts of the website the crawler is disallowed from accessing (using the 'Disallow' directive) and, optionally, which parts they are allowed to access ('Allow' directive). It can also specify the location of a site map using the 'Sitemap' directive. Crucially, 'robots.txt' is publicly accessible, meaning malicious bots or determined users can still access disallowed areas. Therefore, it should never be used for securing sensitive information, only for managing search engine indexing and traffic load.