A sitemap (or site map) is a tool to help search engines crawl and index pages on a website more effectively. That can mean search engines will index more of the pages on your site, more quickly than without a sitemap. That in turn makes it easier for people to find your website.
Google and other search engines will spend only a limited amount of time crawling your site. A complete XML (Extensible Markup Language) sitemap can direct search engines to your most important pages, increasing the likelihood that these pages will be indexed and then discovered by the people you want to attract. This is called "crawl efficiency," and an XML sitemap can improve yours.
Most websites need only a simple HTML sitemap, which has the benefit of helping people, as well as search engines, find individual web pages. If a site is large or complex, or if you want to add more information to a web page listing, Google recommends having both an HTML sitemap and an XML sitemap following the sitemap protocol that major search engines adhere to.