A sitemap is a file where you can list the web pages of your site to tell Google and other search engines about the organization of your site content. Search engine web crawlers like Googlebot read this file to more intelligently crawl your site.
Also, your sitemap can provide valuable metadata associated with the pages you list in that sitemap: Metadata is information about a webpage, such as when the page was last updated, how often the page is changed, and the importance of the page relative to other URLs in the site.
You can use a sitemap to provide Google with metadata about specific types of content on your pages, including video and image content. For example, you can give Google the information about video and image content.
Sitemaps also help to define the canonical page when addressing the issues of duplicate content. If you have a single page accessible by multiple URLs, or different pages with similar content (for example, a page with both a mobile and a desktop version), you should explicitly tell Google which URL is authoritative (canonical) for that page. If you do not do this explicitly, Google will make the choice for you, or might consider them both of equal weight, which might lead to unwanted behavior, as explained in the next section.