A directive telling search engines not to include a page in their index — useful on purpose, damaging by accident.
A noindex directive (via a meta tag or HTTP header) keeps a page out of search results. It's correct on pages like a thank-you page. It's devastating when left over from a staging environment or a template default on a page you actually want indexed.
Because a noindexed page looks completely normal to a human visitor, this is one of the most common silent reasons good pages never appear in search.