A sitemap is a file, usually at /sitemap.xml, that lists the pages on your site so Google and Bing know which addresses exist and when each one last changed.
A sitemap is a file that lists the pages on your site, usually at
yourdomain.com/sitemap.xml. It tells Google and Bing which addresses exist
and when each one last changed. Most site builders and frameworks generate one.
For a new site it matters more than it will later. Google finds pages through links and sitemaps, and a new site has few links pointing at it.
You submit it once in Search Console's Sitemaps report, and in Bing Webmaster Tools. Google's help is clear that a listed page is not guaranteed to be crawled or indexed. Setting it up is covered in how to set up Google Search Console. See also robots.txt.