rpi-webrtc-streamer | providing something like Web Cam server

A signalling server is basically a service that sends the ICE candidates betweens the peers of your conversation. Usually it uses Websockets for this communication. The ICE candidates may include the CoTurn server credentials you provide to the WebRTC Object in JavaScript. But you need to share all candidates between the participants and for THIS you need the signalling server. You can use any language that supports full websockets communication like NodeJS or Java (not PHP!).

Take a look at this article, it describes this very well: https://www.html5rocks.com/en/tutorials/webrtc/infrastructure/#how-can-i-build-a-signaling-service

Except you explicitly want to use Peer-To-Peer WebRTC, I recommend you also to take a look at the tutorials of Kurento Media Server to get a better understanding of this principle, the NodeJS/Java is signalling between Kurento and your browser. Please note by using a media server it will be always in the middle between the participants, what has advantages like reducing the network usage of each participant + recording the whole conversation on the media server, but also disadvantages like no end-to-end encryption.

NodeJS example: https://doc-kurento.readthedocs.io/en/6.14.0/tutorials/node/tutorial-one2one.html

Java example: https://doc-kurento.readthedocs.io/en/6.14.0/tutorials/java/tutorial-one2one.html