Stun server - Aib0t/Bluriest GitHub Wiki
Stun server stands for Session Traversal Utilities for NAT
https://en.wikipedia.org/wiki/STUN
and the easiest explanation can be read here https://stackoverflow.com/questions/45399447/understanding-stun-working
What stun server does in blur:
In order to enable client to client connection, since Blur used p2p connection type, NAT should be traversed. This is done by stun server. Currently, it's unknown what stun server does in order to do it and my stun implementation is not doing the job right.