Forward Error Correction (FEC) is a technique used in networking to enhance data reliability and performance, particularly over unreliable or lossy networks.
Forward Error Correction (FEC): In Cisco SD-WAN, FEC helps in mitigating the effects of packet loss and service degradation by encoding redundant data, which can be used to reconstruct lost packets. This ensures that even if some packets are lost during transmission, the data can still be accurately reconstructed at the destination.
Addressing Performance Degradation: FEC specifically addresses reduced application performance degradation caused by packet loss and network impairments. By providing this error correction capability, FEC helps maintain the performance and reliability of critical applications over potentially unstable network paths.
:
Cisco SD-WAN Security and Quality of Service Configuration Guide
Cisco SD-WAN Forward Error Correction Technical Whitepaper

https://www.cisco.com/c/dam/en/us/td/docs/routers/sdwan/configuration/config-18-4.pdf#page=546

Reference:
https://sdwandocs.cisco.com/Product_Documentation/vManage_Help/Release_18.3/Configuration/Templates/VPN_Interface_Ethernet