FCrDNS, or Forward Confirmed reverse DNS, is when an IP address has forward and reverse DNS entries that match each other. For FCrDNS verification, first a reverse DNS lookup is done to get a list of PTR. Then for each domain name mentioned in the PTR records, a regular DNS lookup is done to see if any of the A records match the original IP address. If there is a forward DNS lookup that confirms one of the names given by the reverse DNS lookup, then the FCrDNS check passes.

IP address 1.2.3.4 resolves to mail.domain.com. 

Host name mail.domain.com resolves to IP addresses 1.2.3.4 and 5.6.7.8.
Thus, reverse DNS for IP address 1.2.3.4 is forward confirmed.

You can check it there: http://multirbl.valli.org/fcrdns-test/