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.
When should I use "noindex" meta tag?
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.