FD.io is a very fast userspace networking library, which allows creating programs for packet processing. While DPDK allows fast read and write access to the NICs, FD.io is focussed on processing the packets. Possible use cases are a packet forwarder, implementing a NAT or a VPN.

More details also in this APNIC blog post: https://blog.apnic.net/2020/04/17/kernel-bypass-networking-with-fd-io-and-vpp/