The ‘noindex’ request to search engines signals that you prefer not to have the URL with this attribute indexed in the search engine.
This is not a command to search engines, so the decision to honor or ignore this request is completely up to the search engine in question. In my experience, Google does honor this request, but be aware that Google may have indexed the URL before the condition was set, so may have the URL and contents in its cache.
Assuming the request is honored, the URL with this attribute will not gain any rank and will not contribute to the website’s overall SEO profile.
A common error is to leave the pages in a website set to noindex after being set ‘live’ post development.
A Google Search Console Fetch command will fail if the URL is set to noindex, so this is a simple test to see if Google will access the URL.
The Robots noindex can be set in place at directory or folder level as well via the robots.txt file.