If you want to play in the official AfterShock leagues or cups, or you want to appear in open ladders and rankings, you need to setup your ingame auth. To do so, go to your openarena-folder and open the aftershock folder. Create a file named autoexec.cfg, if you have that file already open it.
Login tn the AfterShock-site using your username and password. In the user-menu on the left side click on "my profile". Click on "Edit profile" on the top of your profile page. On this page you can find a section named "Set auth password", copy the lines behind "Copy these lines into your config to log in:"
seta aftershock_login "MyUsername" seta aftershock_password "MyPassword"
and add them to your autoexec.cfg. Save the file and you are ready to use the ingame-auth.
If someone got your password(maybe because you shared your config) you can generate a new password. Login to the Aftershock-site, go to "my profile" and click on "Edit profile". Search the "Set auth password" section and click on "Create new password". Follow the steps above to set the new password in your autoexec.cfg.