Noindex is an instruction that asks search engines not to include a page in their search results. It comes in two forms: a meta robots tag in the HTML or the X-Robots-Tag HTTP header. The header also works for files without HTML, such as PDFs. Google explains both in Block search indexing with noindex.
Example
<meta name="robots" content="noindex">A noindex is often exactly what you want, for example on a thank-you page after an order or on internal search results.
Common mistake
Also blocking the page in robots.txt. Google has to fetch the page to read the noindex. If the page is blocked, Google does not see the instruction, and the URL can still appear in the results if other pages link to it.
A second mistake: a noindex left in place after the build or a test phase. A page you want people to find then disappears.
How do you check it?
Use URL Inspection in Google Search Console to see which HTML Google received. The Page indexing report lists pages where Google found a noindex. If a page is still missing from Google after you remove the noindex, it has often not been recrawled yet; you can request that through URL Inspection. More causes are covered in Why is my page not showing in Google?