For a game server to be seen online it normally has to register itself on a master server list. Valve maintains a master server for many different games. The master server is used to populate the server browser.
LinuxGSM can check that supported game servers are listed on the steam master server. This is useful for confirming that the server is actually registering and to help diagnose issues.
You can check that your server is listed on the master server by using
Master server: listed