Definition
noindex is an instruction that tells search engines not to show a page in their results. You can set it as a meta tag in the page's HTML, or as an HTTP response header, which also works for PDFs and other files.
<meta name="robots" content="noindex">
X-Robots-Tag: noindexWhen to use it
Use it for pages that should work for visitors but not appear in search: thank-you pages, internal search results, staging copies, thin filter pages. It is not the tool for duplicates. For those, point to the main version with a canonical URL.
The robots.txt trap
A crawler can only obey noindex if it can fetch the page. If robots.txt blocks the URL, the crawler never sees the tag, and the page can still be indexed from links elsewhere. To remove a page, allow crawling and let the noindex be read. Once the page is recrawled, it drops out of results.
Where to see it
In Search Console the page indexing report lists URLs as "Excluded by 'noindex' tag" and flags submitted URLs that carry the tag. URL Inspection shows the status of one page. A common accident is a noindex left over from staging, so check important pages after a launch.
Ask mowteng
insights_index_audit inspects the URLs in a sitemap and reports those Google has not indexed. See the index coverage audit.
Audit the URLs in my sitemap and list every page that Google has not indexed, with the reason for each.