what is STUN stun.l.google.com:19302 used for
STUN servers are used by both clients to determine their IP addressas visible by the global Internet.If both the peers are behind thesame NAT , STUN settings are not needed since they are anywaysreachable form each other . STUN effectively comes into play when thepeers are on different networks.
As we know that webRTC is peer to peer and the ice candidates aremandatory in webrtc. ICE functionality can be in any of the two ways, STUN and TURN .
There are many stun servers provided by google and other sites whichone could use . You can also setup your own STUn server according torfc5766.
Hope that gives a zest of what and how of stun .