Apple TV/FM Radio System - Manual (Page 96)

4 Click Lock.
The channels you selected are locked. Small locks appear at
the left of the locked channels’ numbers in the channel list.
You can watch a locked channel by typing its number on the
keyboard. A dialog box appears:
If you click Yes and no password is in effect, you tune in to
that channel. However, the channel remains locked and out
of the normal scanning sequence. If a password has been set,
you must type the password before the channel can become
available.
To unlock a channel, choose Channel Setup from the Setup
menu, select the channel, and click Unlock.
89 Customizing the TV Tuner Settings