WebRTC negotiates a peer-to-peer protocol which operates like client/server but has issues with NAT/PAT and firewall traversal. Each peer will realistically only know about its local network and there will also likely be limitations on the peers even being able to communicate with one another directly.
STUN and TURN allow the negotiation to include services outside the local network so that you can relay signaling and stream data to a set of endpoints that both peers can use to communicate with one another. This could be the peers directly or it could be a set of 3rd party servers run by Google or someone open to the public and you can't assume any particular network topology.
STUN and TURN allow the negotiation to include services outside the local network so that you can relay signaling and stream data to a set of endpoints that both peers can use to communicate with one another. This could be the peers directly or it could be a set of 3rd party servers run by Google or someone open to the public and you can't assume any particular network topology.