The robots.txt file is a simple text file used by websites to communicate with web crawlers and bots. It tells these automated programs which parts of the site they are allowed to access and index. By specifying rules in this file, website owners can control how their content is discovered and displayed in search engines. When a web crawler visits a site, it first looks for the robots.txt file in the root directory. If the file is present, the crawler reads the instructions to determine which pages to ignore or follow. This helps protect sensitive information and manage server load.