What the free site check actually measures
The check fetches your page the way a search engine would, with no browser extensions, no logged-in session and no JavaScript rendering assumptions, and grades what comes back. It looks at the title tag and meta description, the heading structure, whether the page is indexable, whether robots.txt and sitemap.xml exist and agree with each other, whether HTTPS is configured properly, how long the server takes to respond, how heavy the HTML document is, whether images have alt text and dimensions, whether structured data is present, and whether the page is usable on a phone.
Each result is a pass, a warning or a fail, with the evidence that produced it. There is no proprietary score hidden behind the number — every point is traceable to a specific check you can verify yourself.