Some of them can also crash the game if you get too crazy, so experiment a bit and do your research before you go nuts with it! Some of these console commands take some fancy know-how of variables and basic programming, as they'll send scripts into action. Now you can open the console by pressing the `/~ key (the tilde key next to the number 1). Click "Set Launch Options" under the General tab.
To enable console commands in Crusader Kings 3, you'll need to run the game in debug mode.