robots.txt
A plain text file at the site root telling crawlers which parts of the site they may request.
What it means
It is the first file most crawlers ask for. It groups rules by user agent, uses Allow and Disallow lines, and can declare where the sitemap lives. Longest matching rule wins between Allow and Disallow, which is the part most people get wrong.
The part a short definition leaves out
robots.txt controls requests, not indexing. A blocked URL can still appear in results if enough other sites link to it, because the crawler never fetched it to see the noindex directive you also added. If you want a page out, let the crawler in and use noindex.
Where this comes from
See whether this is costing you anything
The inspection reports on this signal directly, alongside thirty-seven others, and tells you whether it is a quick fix on your site or something deeper.