The meta robots tag is an HTML element in the <head> of a page. It gives search engines instructions about that one page: may it appear in the results, may its links be followed, may a text snippet be shown with it. With name="robots" the rule applies to all crawlers; with name="googlebot" it applies only to Google. Google describes every rule in its robots meta tag specification.
Example
<meta name="robots" content="noindex, nofollow">This page may not appear in search results and its links are not followed. Other rules Google supports include nosnippet (no text snippet), max-snippet (maximum snippet length) and max-image-preview (size of image previews). When two rules conflict, Google applies the more restrictive one.
Common mistake
Adding a meta robots tag to a page while blocking the same page in robots.txt. A crawler only reads the tag if it is allowed to fetch the page. If the URL is blocked, Google never sees the noindex.
How do you check it?
View the page source and search for robots. Check the HTTP header too: the same rules can sit in the X-Robots-Tag, which you will not see in the HTML. For this and other reasons a page can be missing from Google, see Why is my page not showing in Google?