Add a password to your game when you click on Host Game, and it will be private - an opponent will need that password to join your game. If you have set a password, the Tracker will show it, but only on your machine. Others will not be able to see it.
You are always hosting a game when you are in the Tracker window, even if you do not click on Host Game. Clicking on Host Game will allow you to register a tagline with the Tracker, and give you the option to set a password for your game.