What I do know is that it’s not a predestination like the JW’s. It’s not where 144,000 people are saved only and the rest are doomed. Nor is it a Calvinistic predestination.
It’s far more complicated than that and I think it treads somewhat on the fact that God is omniscient and omnipotent and transcends time.
I’ll let you make sense of this non-Catechism explanation: http://www.newadvent.org/cathen/12378a.htm