Everything that can stop a page being indexed
A robots meta tag with noindex, a googlebot meta tag, an X-Robots-Tag header sent by the server, a robots.txt rule that blocks crawling, an error status code, or a canonical tag pointing elsewhere. This tool checks all of them at once.
Note that robots.txt blocks crawling, not indexing: a blocked page can still appear in Google without a description if other sites link to it. To keep a page out of Google, allow crawling and use noindex.
When noindex is right
Use noindex for pages that shouldn't appear in search: thank-you pages, internal search results, staging copies and login pages. Make sure it's not left on pages you want to rank, which is a common mistake after a site launch.