Noindex
Noindex is a directive telling search engines to keep a page out of their index. It is applied through a meta robots tag or an X-Robots-Tag header. It is useful deliberately and expensive when applied by accident to important pages.
Why it matters for your rankings
Noindex is the most destructive single line in SEO when it goes wrong. A staging site launched with a site-wide noindex left in place will disappear from search entirely within weeks, and nothing else you do will matter until it is removed.
Used deliberately it is valuable. Thank-you pages, internal search results, thin tag archives and duplicate filtered views all belong out of the index. The critical detail people miss is that a page must be crawlable for its noindex to be read. Blocking a URL in robots.txt and adding noindex at the same time means Google never sees the directive, so the page can remain indexed indefinitely with no description. The two controls solve different problems and interfere with each other.
How to check it on your site
Audit every template for noindex
Crawl with Screaming Frog and filter Directives for Noindex. Confirm each one is intentional.
Check the HTTP headers too
X-Robots-Tag headers are invisible in page source. Inspect response headers in DevTools or via curl.
Verify after every deploy
Staging noindex reaching production is one of the most common and costly release errors. Automate a post-deploy check.
Never combine with a robots.txt block
If you need a page deindexed, allow crawling so the directive can be read.