A TURN server is a network entity in charge of relaying media in VoIP related protocols. This includes SIP, H.323, WebRTC and other protocols.
When you try reaching out directly from one browser to another with voice or video data (sometimes other arbitrary kind of data), you end up going through different network devices. Some of these devices include Firewalls and NATs (Network Address Translators) which may decide due to internal policies not to pass your data.
When there are some network securities like firewall, then data packet does not transfer and we do not get proper streaming of another user.
So we use TURN server for this solution.
Ant Media Server does not require TURN server even if there is Symmetric NAT. However it's required if UDP ports are blocked for any reason or Ant Media Server is used as signaling server in P2P communication.