Imagine time as a solid object.
Imagine this solid object was created by an explosion of matter that turned into a web-like block of strands. If you repeated this explosion, the results would be strands that were in different locations than the last time. Each explosion is completely unpredictable.
Imagine the multiverse as every possible explosion of matter in this form. Each strand is a life.
Our four-dimensional unit is one particular result; time is simply us moving through those strands from the center of the explosion to the outer perimeter. We might not know what direction is coming next, but the way the strand moves is already set in stone.
But consider for a moment, that we live in a kind of Twilight Zone, where each one of us moving through those strands is a free-willed person. Everything is fine and dandy as long as our decisions go along with the route. But the moment we start trying to think outside the box, so to speak, we're going to be met with failure and massive amounts of almost inexplicable resistance from the world around us, because no matter how hard we try, we can't deviate from the path of the strand.
That's the Black Iron Prison.