both STUN and TURN server help establish (webrtc) connections between clients in different networks. STUN server just makes some lightweight operations with low CPU and bandwidth requirements this is the reason there are free public STUN servers e.g. from Nextcloud. TURN server relays all the traffic between two clients which can consume many resources (and might become privacy issue) - for this reason there is no (I’m not aware of any) free TRUN servers.