Noindex is an instruction in a page's code, or its HTTP headers, that tells Google to leave the page out of search results even if other sites link to it.
Noindex tells Google to leave a page out of its results. When Google reads the page and finds it, Google "will drop that page entirely from Google Search results", even if other sites link to it. It goes in the page's head:
<meta name="robots" content="noindex">
or in the response as the header X-Robots-Tag: noindex.
Google has to read the page to see the instruction, so a page that is also blocked in robots.txt can stay in results.
A noindex left on from before launch is one of the first things to check when a page will not appear. URL Inspection names it; see how to check if a page is on Google.