I knew someone who had this... I don't really know how to describe it to someone who isn't already familiar with the concept in a way that isn't going to sound fake. Although maybe you will believe it, because it's conceptually not that different from the time crystals at the Boreth Monastery. He had this stone that, used correctly, allowed him to see all those timelines. Every future we might have from a given moment onward. Not all at once, but in rapid succession. (Absolutely insane to watch, by the way.)
Every decision you might make, every unintended consequence of those decisions. Every random (or not random) event that could completely change the course of history. Billions of you's in billions of timelines leading billions of different lives on billions of earths (or not earths) in billions of galaxies.
I never got around to asking him if it changed the way he looked at the world. If seeing every life you might live takes some of the meaning out of actually living them. Conceptually we can believe in the existence of a multiverse, but we generally don't get to see them. I wonder if that makes the universe feel like a cold place. Just tiny data points in the infinite clutter of everything. Or if it makes you cling harder to the life you have, because you know everything could have been completely different.