Account Guard is not the name of a two-bit villain in Champions Online or a featured episode in Star Trek Online but instead a new security system that Cryptic Studios is installing for its games and websites. The system promises to track which computers you've logged in from and block unauthorized access to your accounts.
If your account is compromised, the system will slam the doors closed on your games and send you an email with all of the details. The email also contains a one-time unlock code. This code allows players to open their accounts in the event of a mistaken intrusion.
This system sounds very similar to Trion Worlds' coin lock mechanism. Players have a choice whether or not to participate in Account Guard, as they can opt out of the service through the account page. The studio has posted a FAQ with more answers to particular scenarios.