indexability checker.

Six of the usual suspects, in one pass: whether the page returns a healthy 200 OK, whether it redirects somewhere else, whether there is a noindex in the response header (X-Robots-Tag) or in the page's own HTML (the robots meta tag), whether the canonical points at a different URL, and whether robots.txt tells Google to keep out. Any one of these can quietly keep a page out of the index. For the full picture, see why isn't my page indexed?

This tool checks the technical blockers you control. It cannot tell you Google has chosen not to index a page it is allowed to: thin content, a near-duplicate of another page, or simply low priority. That call lives in Search Console's URL Inspection tool, not here. The other likely answer is time: a page still has to be crawled, rendered and indexed, so a new or newly-fixed page may just not have been picked up yet. Use the check if this page is on Google button after a check to see whether it is actually in the index right now.

It is a noindex instruction sent in the server's response header rather than written into the page. Because it never appears in the HTML, you cannot spot it with view-source or by reading the page, which makes it one of the most common hidden reasons a page vanishes from the index. It is usually set in a server config, a CDN rule, or a plugin. This checker reads it because it fetches the real response, headers and all.

No, and the difference trips a lot of people up. robots.txt controls crawling (whether a bot is allowed to fetch the page). noindex controls indexing (whether it can appear in results). Blocking a page in robots.txt usually keeps it out of the index as a side effect, but not always. Worse, if you block a page in robots.txt, Google cannot crawl it to see a noindex tag, so the two can cancel each other out. If you want a page gone, let Google crawl it and serve a noindex. The robots.txt reference has the full rules.

It tells search engines which URL is the master copy when similar pages exist. If this page's canonical points at a different URL, you are telling Google to index that one instead, so this page will usually stay out of results on its own. That is exactly what you want for a duplicate; it is a problem when a page that should stand alone accidentally points elsewhere.