"Non indexable pages" are pages on your domain that can't be found in search engine results. There are multiple possible causes why this might be the case.
For example, this might result from a directory or specific URLs being excluded from indexing by your robots.txt.
2. Canonical Tag
Another common reason is the canonical tag leading to a different page. The canonical tag defines the page which should be indexed. So either the canonical should point to itself or to a page with the content you want to be indexed. You can find more information on the canonical tag here: RYTE Wiki Canonical Tag
The redirect enables webmasters to define another URL address where the content of the page can be found. This is used when old URLs are not available anymore. The most common redirects are status codes 301 and 302. You can find more information on status codes here: RYTE Wiki HTTP Status Code.