A sitemap.xml file is a document that provides a list of all the pages on a website to help search engines crawl and index the site more efficiently. It is typically located in the root directory of a website and follows a specific XML format. The sitemap.xml file includes URLs, metadata about each URL (such as the last modification date and change frequency), and information about the site's structure. It is important for SEO purposes as it helps search engines understand the organization of a website and discover all its content.