It is placed in the head of the page as a meta robots element with the value noindex, or sent as an X-Robots-Tag HTTP header. For it to work, the page must remain crawlable, because a crawler blocked by robots.txt will never see the instruction. Noindex is the correct tool for pages that need to exist but should not appear in results: thank you pages, internal search results, duplicate print versions, staging content, gated downloads and thin tag archives. Pairing it with follow lets link equity continue to flow through the page's links.
The risk is leaving it somewhere it does not belong. Development sites are usually built with site wide noindex enabled, and when the site goes live without that setting being removed, the whole thing stays invisible while everyone wonders why rankings never arrived. It is one of the most common reasons a brand new contractor site gets no traffic for months. After any launch, view the source of your homepage and a service page and confirm there is no noindex, then verify in the Search Console page indexing report.
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