It refers to human free will and its relationship to the future. It teaches human freedom and that our choices ahead of time cannot be known by God. If God knows our future choices, the question they ask is, how can we be truly free? We would not choose what God knows what we would choose; that is, the attribute of His omniscience and sovereignty are denied and it means if He knows the future decisions of every man we would not be free.
The Bible teaches that God is greater then our hearts and knows all things (1 John 3:20) and Saint Peter also testified that Jesus knew all things (John 21:17).