Anything predetermined has been decided or set up ahead of time. When you sit down to dinner with your family at a predetermined time, it means you all discussed when you'd be eating long before the meal was ready.
If you learned that the outcome of a presidential election was predetermined, you'd probably be pretty angry — and you wouldn't be likely to vote, since the winner would have already been decided. This adjective (and its accompanying verb, predetermine) originally had a religious meaning, the idea that God has already decided the path your life will take.