TURN or “Traversal using Relay NAT” (TURN) protocol is a STUN extension that allows clients to send and receive data through an intermediary server.
TURN Servers are used when a peer connection needs a supplemental service to punch through specific types of firewalls or restricted networks. Eg: When endpoints are stuck behind different types of NATs, or when a symmetric NAT is in use.
TURN gets through these obstacles by relaying the media. Clients connect to the TURN server, rather than trying to connect through difficult NATs, resulting in improves connection success rates at a global scale.
For more information about how TURN works, see our Blog Post: ICE and WebRTC: What Is This Sorcery? We Explain