An XML sitemap is a behind-the-scenes list of all the pages on your website, made for search engines. It helps Google find and understand everything you've published, so nothing important gets missed.
Also known as: Sitemap.xml, sitemap index, search engine sitemap
In plain English
An XML sitemap is a machine-readable file (usually at yoursite.com/sitemap.xml) listing every page you want indexed, with hints about how often they change. It's a map you hand Google so it doesn't have to guess what exists on your site.
Why it matters for your business
It doesn't boost rankings directly, but it helps Google discover and index pages - especially new content, deep pages, and anything not well-linked internally. Most SEO plugins generate and maintain it automatically.
When you'll run into it
It should exist on every site and be submitted in Google Search Console. Check that new pages appear in it and that old/removed pages drop out.
Run into this on your site?
Speed Optimization