What is the noindex tag and is it on my pages by accident?

The noindex tag is an instruction in a page’s code that tells search engines to keep that page out of their results entirely - useful on purpose, disastrous by accident:

• What it does - a page with noindex is crawled but never shown in search results, no matter how good it is.
• Where it belongs - thank-you pages, internal search results, staging sites and admin pages you genuinely want hidden.
• How it ends up there by mistake - a staging site launches with its noindex still in place, or a content system applies it site-wide and nobody notices.
• How to check - Google Search Console’s Page Indexing report lists pages excluded by noindex; that is the first place to look if traffic vanished.

If a page that should rank is invisible in Google, check for a stray noindex tag before assuming anything else is wrong.