A machine-readable file listing URLs a site wants search engines to discover, with optional supporting information.
A practical example
A publisher lists canonical, indexable article URLs and keeps modification dates accurate when content changes materially.
What to watch for
Submitting a sitemap is a discovery hint, not a promise of crawling, indexing or rankings. Keep broken, redirected and excluded URLs out of the intended indexable set.
Related reading
Explore Site Map and Indexing. Try our noindex checker for a practical next step. See how this fits into Technical SEO.