DO NOT edit the gamemode_competitive.cfg like the other answer suggests, because it is overwritten every time you update the server.
Valve documentation explicitly says
Admins should not modify the gamemode_XXX.cfg files
Instead create a gamemode_XXX_server.cfg and place your overrides there.
That string is a SteamID for a Steam Datagram Relay enabled server. It is intended to hide the public IP address from players on the server, along with some other benefits, such as authentication, encryption, rate-limiting, protection from DOS attacks, and in some cases better ping.
Support for SDR was added to CSGO in 2016 according to this Reddit post by a ...