Until information about an object is known, i.e. a measurement is made, either by a person, a machine, whatever, anything that can determine something about the object, then it exists as a wave, a superposition of all possible states it can be in. When the measurement is made, for example the particles position is determined somehow, it can't at that point exist as a superposition of all possible states, that's what they mean by collapsing the wave function. It goes from "it's possibly anywhere" to "it's specifically here, right now" and as a result it behaves like a single, discrete object, not a range of possible states.
The double slit experiment just demonstrates this, the superposition means it's possible the object goes through both slits at the same time, because nothing is known about the object to determine that it can't be doing that. Imagine that to go through either left or right slit is 50% each, if you checked at a any given time. Since no one can say which for sure all possibilities are being expressed, so it's effectively going through both at the same time, as a result you get an interference pattern. You check and find it goes through the either left or right, 100% sure because that's where you find it, at that point it's then 0% possible to go through the other slit, it can't be somewhere it's defined not to be. The pattern you see reflects this change in behaviour, 2 bands, the only 2 possibilities that exist for informationally known particles to reach the screen.
It's got nothing to do with the universe being a simulation or anything like that. The craziest thing from it all is that at the quantum level everything exists as a state of probabilities, the concept of solid matter doesn't exist because even things that were thought to be solid, like electrons, exhibit wave like properties. But all that is counter intuitive because at our life scale that's not how it behaves, in other words, the wave functions at our scale are collapsed quite sufficiently to say our reality is real enough.
Hope this helps a bit but it is somewhat tough to explain.