Signaling server

The essential intermediary that enables two devices to find each other and negotiate a direct WebRTC connection.

Signaling servers act as the traffic controllers for real-time communication. While WebRTC handles the actual peer-to-peer data flow, it cannot initiate a connection without an external handshake. The signaling server facilitates this by exchanging Session Description Protocol (SDP) offers and ICE candidates (network addresses) between clients. Whether you are deploying a simple Node.js app using Socket.io or a robust production environment with Matrix or XMPP, the server stays out of the media path once the connection is established. It is the critical first step for every video call, file transfer, and multiplayer game session.

https://developer.mozilla.org/en-US/docs/Web/API/WebRTC_API/Signaling_and_video_calling