What is an XML sitemap?
A machine-readable list of a site's pages for search engines. It helps them find everything; it does not make anything rank.
An XML sitemap is a file that lists the pages on your site for search engines, usually with the date each one last changed. You submit it in Google Search Console and Google uses it as one of the ways to find your pages.
It is useful, and every site should have one. It matters most for new sites with few links pointing at them, for large sites with pages buried deep, and for pages that change often.
It is also one of the most overrated things in SEO. A sitemap helps Google find pages. It does not make Google index them, and it does not make anything rank. A page that is in the sitemap and linked from nowhere else on the site is telling Google it exists while telling it, through the lack of links, that it does not matter. Good internal linking does far more for discovery than the sitemap does.
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