What does 'Noindex page' warning in Site Audit mean?

Once you've run a crawl of your site in Site Audit tool, you might see a 'Noindex page' warning in the report under Internal pages menu:

It means that these are the pages of your site with a "noindex" meta tag added.

A "noindex" meta tag is used on a page in order to prevent it from search indexing. 

Including a noindex meta tag in a webpage’s HTML code tells Googlebot to entirely drop that page from its index, preventing it from appearing in search results.

You should only use this tag on the pages you don't want to appear in search results.

If you want a page to be indexed by search engines, you should remove this tag.

Be sure to check Google's guidelines on the implementation of this tag here - https://support.google.com/webmasters/answer/93710?hl=en

Please take notice also that for the noindex meta tag to be effective, the page must not be blocked by a robots.txt file. Otherwise the search crawlers will not be able to see the noindex tag.