Direct Answer
An XML sitemap is a file (sitemap.xml) at the root of a website that lists every URL the owner wants search engines to index. Yes, every business site needs one. Submit it to Google Search Console and Bing Webmaster Tools. Sitemaps speed up indexing of new pages from days to hours.
Voice answer (≤30 words)
“An XML sitemap lists every URL you want indexed. Every site needs one and you submit it to Search Console and Bing Webmaster Tools.”
Split into sub-sitemaps for sites over 50k URLs (Google's per-sitemap limit) — index sitemap + multiple section sitemaps.
Add lastmod dates so Google can re-crawl only what changed since the last visit.
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