What is an XML sitemap and do I need one?
An XML sitemap is a machine-readable list of every page on your site with the date each last changed. Search engines can find your pages without one by following links, but a sitemap helps them find new pages sooner and not miss any. WordPress generates one automatically at /wp-sitemap.xml, and every site should submit its sitemap in Google Search Console.
Why it matters
Google discovers pages by following links, so a page that nothing links to, or a new page that has not been linked yet, can go unnoticed for weeks. The sitemap hands Google the full list. It also tells Google when each page last changed, so updated pages are re-crawled sooner, and it is the quickest way to get a brand new site or a new section indexed. For sites with hundreds of pages it is close to essential.
What belongs in it
Every page you want listed in search, and nothing else: no thank-you pages, no tag archives with one post, no draft or private pages. A sitemap full of thin pages dilutes the signal. Most generators handle this sensibly by default.
How to check
Our check looks for a Sitemap line in your robots.txt and tries the common addresses /sitemap.xml and /wp-sitemap.xml. In Google Search Console, the Sitemaps report shows whether Google has one and when it last read it.
How to fix it
On WordPress, the built-in sitemap at /wp-sitemap.xml is on unless an SEO plugin has replaced it with its own, usually at /sitemap_index.xml. Either is fine. Add a Sitemap line to robots.txt pointing at it, then submit the same address in Search Console. On other platforms, most generate a sitemap automatically; check the platform's help for the address. If nothing generates one, a free online generator can build a static file, though it will need regenerating when pages change.
Not sure how your own site does on this? Run the free check: it takes about twenty seconds and reports on this and thirty other things.