This means the listening socket was created using SOCK_RAW as opposed to SOCK_STREAM or SOCK_DGRAM. Raw sockets are used for working with ICMP, doing packet sniffing, sending some types of custom TCP packets, etc. Basically anything that isn't UDP or TCP, you'll need a raw socket for.
Historically programs like `ping` and `traceroute` use raw sockets. Using raw sockets requires privilege, which is why those historically have been set-uid on Unix systems.
