Resources:
- https://partner.steamgames.com/doc/features/multiplayer/networking
- https://partner.steamgames.com/doc/features/multiplayer/steamdatagramrelay
Steam Networking is a bundle of APIs that do a lot of things and most notably support Steam Datagram Relay, which is provided by Valve to proxy connections to your server/SRCDS, to protect your server/SRCDS from DDOS, and provide P2P support.
Previously, if you could not port-forward your SRCDS server's ports, it would only work as a LAN server and other players wouldn't be able to join.