What is this tool?
Cloudflare is a Content Delivery Network service provide as well as acts as a layer of protection for the website Wikipedia.
When used, the original IP address of the Cloudflare protected IP is hidden and proxified with the Cloudflare's IP which is highly protected. Cloudflare Resolver resolves all sub-domains for the domain name and searches for the IP address and checks if that's Cloudflare. Any sub-domain which is NOT Cloudflare protected, it is marked by the tool and is shown in the list.
However, the non-cloudflare subdomain may not be the website's origin IP address as many times web developers uses mail-server in non-proxified sub-domains. You can use this tool to check if your website's configuration for Cloudflare is proper or needs more configuration.