The Infinite Forest is basically a pocket dimension. In "Beyond Infinity" we find out that time flows slower in the forest because after completing a ~20 minute mission and leaving the forest, Ikora says we just left. This accounts for the fact that Saint-14 went missing an indeterminent amount of time, more than likely only a few centuries before we revive, but says he's been fighting for thousands of years. When we find the map of the infinite forest, Sagira says she can't track Panoptes because it's changing. That implies that the future doesn't branch off at every decision like the multiverse theory states, but instead follows a branch to its conclusion, then resets to try it again with different choices. The reason why we can stop the futures that are simulated in the forest is because even though Guardians can be simulated (like how Osiris uses his reflections), Light cannot. Ruining the test results of a simulation by doing something that can't be processed is like saying 2=3 without giving a reason why, thus negating the whole purpose of the simulation. The reason we can find Panoptes by following his past to the future is because they're the host of the Reality Engine, as opposed to a guest like us or a simulation like all the enemies. The Infinite Forest can always rewrite what enemies were where and what paths existed (like in the Vex Tech Demo that is the procedurally generated sections), there will always need to be a constant, and that's what Panoptes is. Without a host, new simulations can't be created, which is how we stopped the dark future. Sometime later, however, a new mind hosts the forest, and that's why Osiris stayed in the Forest. TL:DR [spoiler]Panoptes is the host of a Reality Engine that can run simulations of nearly anything in the universe. In the Infinite Forest, time moves slower allowing the Vex to explore exponentially more realities within shorter spans of time. Osiris is basically piggybacking off of Panoptes to see all the futures the Vex want to make, but because Light can't be simulated, Panoptes can track it as an anomaly. Sagira escapes the Forest and brings us, a Guardian with exceptional light, in to help Osiris. Because the Vex can't simulate Light, Panoptes can't predict where we'll be in the Forest, we go through all it's simulations tearing down scenarios and corrupting data. We're basically a Virus in the system.[/spoiler] TL:DR;TL:DR [spoiler]Big Vex sees future while Egypt man spies on Vex. We don't belong because we people, not AI, so Big Vex uses roboracist magic to try to get rid of us. We paper shredder his homework so no one can copy it and we kill him. No one knows his password so no one can delete or add to his history. Computer is useless until someome figures out his password[/spoiler]

<The Infinite Forest is a reality engine. It's essentially a machine that can simulate trillions of realities at once. It is the size of Mercury's core at its base form and takes a shit ton of processing power to use. We are trying to stop Panoptes, the "admin" of the Infinite Forest from figuring out a way to ensure the perfect Vex future. Using the Forest, it can figure out what steps need to be made to make it reality. Unfortunately, just as the plan started to come into fruition, we killed it, therefore erasing all the data on how the plan to make the perfect Vex future would work.>

An inter-dimensional simulation engine designed to predict the future based off of present circumstances. The goal was for Panoptes to manipulate ALL branches so the future, no matter what the present looked like, would always be the same: a future with nothing but the Vex. Why "The Guardian" was so crucial is because Guardians are the one thing the Vex could not simulate. [spoiler]Did I, Albert Einstein, clear it up for you? 🤓😎[/spoiler]

It doesnt...

The Infinite Forest is a simulation engine. The Vex use their vast understanding to create extremely advanced simulations that are near perfect reflections of reality. They then brute force what the future will be like by following these simulations to their eventual conclusion. The Vex find new potential timelines by manually altering factors in these simulations. The Vex then use this information to determine what actions they should be taking.

It doesn't work.