What reverse DNS is
Ordinary DNS turns a name into an address. Reverse DNS goes the other way: for 192.0.2.10 a resolver asks for the PTR record of 10.2.0.192.in-addr.arpa, and for IPv6 for a name built from the 32 hex digits of the address under ip6.arpa. The PTR record is controlled by whoever holds the address block, usually the hosting provider or ISP, not by the owner of the domain. You set it in the provider's control panel or by asking them.
Forward-confirmed reverse DNS and why mail servers care
Anyone who controls an address block can write any name into a PTR record. So receivers check the other direction as well: the PTR name must resolve, through an A or AAAA record, back to the same address. That pair is called forward-confirmed reverse DNS (FCrDNS).
Receiving mail servers use it as a basic hygiene test. An address with no PTR record, or a PTR name that does not resolve back, looks like a residential line or a misconfigured server, and large mailbox providers say in their sender guidelines that sending addresses need valid forward and reverse DNS. A mail server should have a PTR record that names a host in your domain, that host name should resolve to the same address, and ideally the server introduces itself with that name in HELO.
What this tool does
For an IP address it queries the PTR record and then resolves every PTR name forward. For a domain it first looks up the A and AAAA records and does the same for each address (up to eight). The result says confirmed, mismatch, no address or could not be checked for each name. A failed query is never reported as "no PTR record". The tool only sends DNS queries; it does not connect to the address.