A sitemap is a file where you provide information about the pages, videos, and other files on your site, and the relationships between them. Search engines like Google read this file to more intelligently crawl your site. A sitemap tells Google which pages and files you think are important in your site, and also provides valuable information about these files: for example, for pages, when the page was last updated, how often the page is changed, and any alternate language versions of a page.
Visit here to know do you need a sitemap ? Learn about sitemaps | Google Search Central | Google Developers
How to generate sitemap ?
To generate sitemap ‘n’ number of sites are present who will generate sitemap for your site, But you can also create sitemap for your site by just adding sitemap.xml ahead of your site as shown above.
You can also add sitemap in Google Search Console. Watch below video to know more:
Note:- A sitemap must be always with extension .xml do not generate sitemap with extension .html or any other.