A "noindex" tag is an HTML meta tag telling search engine crawlers to no-index the specific webpage.
What is Noindex tag?
The HTML code for a noindex tag is looks like this: <meta name="robots" content="noindex">
When a web page contains a noindex meta tag, As a result, that page won't appear in search engine results pages (SERPs).
You should always no-index to low-value website content or any content you don't want users to find on search result pages. It may be your Admin page and login pages.
The most admin and login pages should have noindex tag of website.