Noindex
Also known as: robots meta, X-Robots-Tag
An instruction, in a meta tag or an HTTP header, telling search engines not to include a page in their index.
Noindex is honoured. That is what makes it useful on a thank-you page and catastrophic on a product category. It can arrive two ways, a robots meta tag in the HTML or an X-Robots-Tag response header, and the header version is invisible in view-source.
Every other signal on a page is decorative while this one is failing, which is why it carries the heaviest weight in the Search Visibility reading.
The header version is the one that catches people. A site can look perfect in the markup and be entirely absent from search because a server configuration is adding one line to every response. It is worth checking with a request rather than a browser.