What gets checked
Indexability first, because nothing else matters while a page is blocked: robots meta tags, canonical tags, and whether the page contradicts itself by carrying both noindex and a canonical.
Then the things that decide how the result looks and reads: the title measured in pixels rather than characters, the meta description, heading structure and whether levels are skipped, image alt text, internal and external link counts, and anchor text that says nothing.
Then the technical basics that break mobile: the viewport tag, disabled pinch-zoom, the lang attribute, character encoding, server response time and HTML weight. Plus structured data — whether it exists and whether it parses, since a trailing comma silently voids the whole block.
Why the explanations matter more than the score
A number out of 100 is a rough guide and nothing more. It has no relationship to where you rank, because ranking depends on competition, links and content quality — none of which any on-page checker can see.
What is genuinely useful is knowing which findings to act on. "No meta description" and "page is set to noindex" both appear as red marks in most audit tools, and one of them is a formatting improvement while the other is why the page gets no traffic at all.
So every finding here carries its reasoning. Some are worth an hour, some are worth a note in a backlog, and a few are genuinely fine to ignore for a given page. Being told which is which is the difference between an audit and a list.
What this cannot tell you
It reads the HTML your server returns and does not run JavaScript. On a React or Vue site that renders content in the browser, this sees the shell. Google does render such pages, with a delay — but if your text only exists after a script runs, that is worth knowing regardless of what any audit says.
It says nothing about backlinks, rankings, competitors or search volume, because those require data no free tool has honest access to. Any tool that reports your "authority score" for free is showing you a number of its own invention.
And it cannot judge whether your content is any good, which is the part that decides most rankings. On-page technical work removes obstacles; it does not create a reason for anyone to rank you first.