It is a meta tag that can be added to the HTML code of a web page; it prevents search engines from presenting the page's content in search results. The crawlers can pass, visit the page, read what the tag contains, but it is not indexed.
There are several cases where a noindex tag should be used. Some of them are:
You do not want final users of a website to see a company's login pages (although there are some exceptions). Using noindex solves this problem.
Although the canonical tag is used to deal with duplicate content, there are cases where noindex should be used.
When you are the only person contributing content to a website, you don't need an author page and it would probably look very similar to the content on the home page. In this case, it should not be indexed.
When there is a website that gives community members the ability to create a profile to contribute blog information or content, this is something you want to regulate.
You only want the best contributions in terms of content to appear in the SERPs since this is a sign of the website's authority and generates a good user experience.
One of the biggest concerns for Google and search engines, in general, is quality content. The best web pages in terms of quality will be favored, while the opposite will happen for those with little or little content.
In these cases, when you do not want this content to act as a representation of your website, but you also do not want to delete that content, it is not indexed.
When you have exclusive content that only members can access, these pages should not be indexed, thus preventing non-member users from viewing your content.
The goodbye or thank you pages, although they are a courtesy and good manners for a customer after making a purchase or visiting the website, they do not require to be viewed in the SERPs. These pages are not indexed.
Provisional versions of a page or under construction pages that are not yet ready to be visited by the public should not be indexed.
To prevent a page from appearing in Google Search, the noindex meta tag is included in the page's HTML code. The next time that page is crawled and the tag is seen, the crawler will exclude that page from Google Search results, even if other sites link to it.
For the noindex rule to be effective, you must make sure that the page is not blocked by a robots.txt file, since if so, the crawler will not take the noindex directive, and the page may still appear in the search results.
In case of not having root access to the server, the use of the noindex directive makes it possible to manage the entrance to the site through the page.
There are two ways to use noindex: as an HTTP response header and as a meta tag. Both fulfill the same function.
You should be aware that the noindex directive can be interpreted differently by some search engine crawlers. There is a possibility that the page will continue to be displayed as a response from other search engines.