An XML sitemap is a roadmap you hand the search engines so they do not have to guess their way around your site. It will not magically rank you, but it makes sure your good pages actually get found.
The short version
An XML sitemap is a file that lists the important pages on your website so search engines can discover, crawl, and index them more efficiently.
What to include
Pages you want indexed
Canonical versions, not duplicates
Live, working URLs only
What to leave out
Skip thin, blocked, redirected, or duplicate pages. A sitemap full of junk just tells engines you do not know which pages matter. Keep it lean, then submit it in Google Search Console.
FAQ
Do I need an XML sitemap?
Most sites benefit, especially larger ones or new sites with few backlinks. It helps engines find pages they might otherwise miss.