What 'indexed' actually means
Google's search engine works in three stages: crawl, index, rank. Crawling is when Googlebot downloads your page. Indexing is when Google decides the page is worth storing in its searchable database. Ranking is where it shows up for a query.
A page can be crawled but not indexed — this is one of the most common (and most misunderstood) reasons small business sites get zero Google traffic. Google saw the page, looked at it, and decided not to keep it. Usually because the content is too thin, too similar to other pages on the web, or because a technical signal told Google to skip it.
If your site isn't indexed, no amount of "SEO work" — keywords, backlinks, schema — will help. Indexing is the gate. This checker tells you whether you're past the gate.