Enter a domain and the tool fetches the homepage, then looks for the site's robots.txt to find any Sitemap: lines it declares. If robots.txt lists sitemaps, those are used; if it does not, the tool falls back to the conventional /sitemap.xml location. It then walks the sitemaps it finds — following a sitemap index to its child files where one exists — and counts the unique URLs listed inside the <urlset> documents. The headline number is that count of indexable URLs.
Alongside the count, the tool checks whether the homepage itself is indexable by reading the robots meta tag in the page head and flagging a noindex directive if present. It reports how many sitemap files it discovered, the robots.txt status, and a sample of the first URLs it found so you can eyeball what is actually listed. This is deliberately different from typing site: into Google. It measures what the site itself declares as indexable, which you control, rather than what a search engine currently has stored.