If you have enabled TrackBack pings to a particular category, you can selectively allow access to that functionality by setting a passphrase for the category. Users who wish to send pings to your TrackBack-enabled category will need to know the passphrase in order to ping you.

This is a good way to enable "private" TrackBack items; even if someone can guess the TrackBack ID, he will not be able to send a ping unless he knows the passphrase.

This is optional. If you do not set a passphrase, everyone will be able to ping you without providing a passphrase.