The reverse Domain Name System (rDNS) is a protocol used to translate the sending server IP address into its host name (host.example.com).
By translating the sending server IP address into its hostname, the rDNS check determines that the IP address is connected to an authentic domain name. rDNS checks also validate that email is not coming from dial-up users or a dynamic IP space, which are methods used by spammers.
During a rDNS lookup, your Simple Mail Transfer Protocol (SMTP) server verifies that your IP address matches the domain name submitted by the SMTP client in the HELO command. HELO is an SMTP command sent by an email server to identify itself when connecting to another email server to start the process of sending an email. It is followed with the sending email server's domain name.
Mailbox provider filtering impact
Mailbox providers check for Forward Confirmed Reverse DNS (FCrDNS) records. Many mailbox providers, including Gmail and Yahoo!, will block email coming from a mail server that does not have rDNS in place.
See the What is Forward Confirmed reverse Domain Name System (FCrDNS)? article for more information and requirements.