Reverse DNS is forward DNS run backwards: instead of "what IP does this hostname resolve to," it asks "what hostname is registered for this IP." It rarely matters for normal browsing, but it's a hard requirement for production mail servers.
The mechanism
PTR records live under the .in-addr.arpa (IPv4) or .ip6.arpa (IPv6) zone, which the IP's owning network operator controls. Setting your own PTR requires asking your ISP / cloud provider; most allow it via a support ticket or control panel.
FCrDNS — forward-confirmed reverse DNS
The receiving mail server resolves your sending IP to a hostname (PTR), then resolves that hostname back to an IP (forward DNS). If the forward resolves back to the original IP, FCrDNS passes. If not, the mail is suspicious. Most major receivers reject mail that fails FCrDNS.
Our Reverse DNS tool runs both queries and reports the FCrDNS result.