The robots.txt file is a text file placed in the root directory of a website, used to guide and manage the behavior of search engine crawlers or robots. It provides instructions about which areas of the site should or should not be crawled and indexed. This can help prevent overloading your server with requests, protect sensitive content from appearing in search engine results, and improve SEO by directing crawlers to the most important content. However, it's crucial to use robots.txt carefully, as incorrect usage can unintentionally block important content from being indexed.