What is Sitemap?

Also called XML sitemap, sitemap.xml.

A sitemap is a file listing every page on your site you want found, so search engines are told directly rather than having to discover each one by following links.

In more detail

It matters most for pages not linked prominently from your homepage, and for new sites with few inbound links. Each entry can carry a last-modified date, which is a scheduling hint — one that only helps if the dates are real. A sitemap that claims every page changed today, every day, gets discounted.

Why it matters for your site

Publishing one and submitting it in Search Console is the shortest path from "my pages exist" to "my pages are known to exist", and it is usually a ten-minute job.