Pieces of the Past
So, as you tell your story, you will occasionally dip into the past to tell the parts before your opening. Each "dip" creates a piece of the past.
If you are content with disconnected pieces, that's not a problem. For example one of my books has four flashbacks to times when the main character learned something from the wise man. The reader can't know the temporal order of those flashbacks, but that doesn't matter. I don't even know the order; there is no structure, just four unconnected pieces of the past.
The Problem of Pieces
If you expect your reader to assemble your pieces from the past into a coherent story, you have a problem. When a story is in normal chronological order, it's simple and natural for the reader to see the scenes and events as occurring one after another. Connections and causal relationships from one event to the next are obvious.
When you dip into the past to create a piece, the resulting pieces usually are not in chronological order. So the reader might have no way of knowing the chronological order.
Content will give clues, but making those inferences is not simple. Remember, the pieces are presented at different times and will not be perfectly remembered.
The time or order of the pieces can be marked, giving the reader the information needed to order them in time. But the marking itself is effort, and the reader still has to work at remembering and assembling.
More generally, the mere fact that the pieces are read at different times makes them difficult to assemble (than if they were read one after another).
For example, The Curse of Chalion has a beautiful moment at the end when he remembers a promise he made to the gods, and now he (and the reader) can see how that makes sense of what has happened to him throughout the story. It's really nice. However, it's the ending moment of a longer event from the past. That whole event is presented in numerous places, so it was difficult (at least for me) to assemble the whole thing to get context.
A Heart in a Body in the World contains numerous jumps into the past, to tell a complicated story of what happened, and the events need to be placed in temporal order and their relationship understood. It's a stress. Probably most readers succeed more or less, but it's work and not crystal clear in my mind.
A common belief seems to be that if authors present events out of chronological order, the reader will assemble them into the correct order and there is no problem.
I once assumed this. It doesn't seem like a plausible assumption to me now, but no one ever questioned or challenged it. I didn't even know I was making that assumption.
In fact, jumping around from time to time is a problem, and assembly of disconnected pieces is a problem. How much of a problem? The author who blithely time jumps probably wants to believe those are essentially no problem. I doubt that's true. But I don't know how much effort those are; all we know logically is that they must be some effort.
However, by way of evidence, I will note this. Usually, sooner or later, an author settles down and tells a story in mostly chronological order. Why? I think that's because it's too hard to read and understand a story when it's told in disconnected pieces with a lot of time jumping.