Robots.txt is a file located at the root directory of a website that tells search engine crawlers which pages or files the crawler can or cannot request from the site. Sometimes, webmasters use the robots.txt file to block certain parts of their site from being crawled by search engines. This could be intentional, such as blocking sensitive or duplicate content, or unintentional, resulting from misconfigurations. If important content is blocked by robots.txt, it won't be indexed by Google or other search engines, effectively hiding it from search results.