What is an X-Robots-Tag?
The X-Robots-Tag is a method of implementing Robots exclusion standards on resources via HTTP headers.
Multiple X-Robots-Tag headers can be used, or they can be bundled together using a comma separated list of directives.
It is possible to target directives at specific bot user agents.
Example
As per Google’s documentation the following is an example of an X-Robots-Tag issuing a noindex command via a HTTP header.
HTTP/1.1 200 OK
Date: Sat, 29 May 2021 21:42:43 GMT
(…)
X-Robots-Tag: noindex
(…)