FCrDNS stands for Forward Confirmed Reverse DNS and it is used to show relationship between an IP address and domain name. To explain this let's look at the following examples:
example.com (domain) → 188.8.131.52 (IP)
184.108.40.206 (IP) → example.com (domain)
FCrDNS means that the above Forward and Reverse DNS must match. In the above case they do so FCrDNS is confirmed. Although a weak form of authentication it is a good practice to have FCrDNS as it is used among many mail providers as an anti-spam method which in turn can help with email deliverability and placement.
See also: Reverse DNS