It refers to the page's possibility of being displayed at the SERPs. A non-indexable page means that it will not be present on search engine results and will end by not bringing traffic to it. You can prevent search engines from indexing a page by using the "noindex" or "none" instruction.
By using the meta tag "noindex" or "none" in the HTML code or with a header "X-Robots-tag: noindex" you can decide which page should be visited by search engines.
If you want to prevent search engines from indexing a page, you can add the "noindex" or "none" instruction as a meta tag or HTTP response header. Both have the same effect and you can choose depending on the degree of control you have over your server and your specific publishing process.
Example - <meta> tag
Add the following meta tag in the <head> section of your page :
<meta name="robots" content="noindex">
Example <meta> tag for Google
Add the following meta tag in the <head> section of your page if you only want to block Google crawlers:
<meta name="googlebot" content="noindex">
Example - HTTP Response Header
You can choose to display an X-Robots-Tag header with a noindex or none value in your response :
HTTP/1.1 200 OK (…) X-Robots-Tag: noindex (…)
If the page is not being indexed when it was supposed to be, you should :
Ps: It may be that the request for non-indexation is voluntary or that it is an error, hence the presence of this analysis.