Noindex is a directive telling search engines not to include a page in their index. Unlike robots.txt (which controls crawling), noindex reliably keeps a page out of results.
Noindex is a meta tag or HTTP header that says "do not show this page in search." Useful for thank-you pages, admin pages and thin utility pages.
Do not block a page in robots.txt if you also want noindex applied — Google must crawl it to see the noindex.
For pages with no search value: internal search results, thank-you pages, duplicates.
It takes effect after Google re-crawls the page.