The subject is Dimensionality vs Multiple Universes.
By dimensions, we mean the physical dimensions of space. 2d, 3d, etc.
you can think of them as degrees of freedom, or the amount of information you need to locate something.
for example, a flat sheet is 2d. like a map, you need only 2 pieces of information to locate something on a map, latitude and longitude. left-right and forward-back.
3d is space. you need a third: up-down. that's the real world we live in. 3d movies add depth to the otherwise flat movie screen.
to get from 2d to 3d, you draw lines at right angles to both 2d dimensions. we can see that to do that, you take a flat picture, and move up. those up lines are at right angles to both left-right and forward-back dimensions of a flat picture.
4d gets interesting. we live in 3d, so we cannot really know it, we can only imagine it by metaphor. if you take a cube, and draw a line at right angles to all 3 dimensions, that would be the 4th. we can't do it, we can't even really imagine it.
but just like a 3d cube can cast a 2d shadow, the 4th dimension casts a shadow on our 3d world, and that shadow is time (that's part of relativity).
we live moment to moment, but our 4d selves is the self made up of adding all the moments of our life together. see each moment added together as one thing, and that's the 4d version of yourself.
5d adds another level of choice. take your lifetime, the 4d version of you. then imagine all the possible ways your life could have turned out. add them all together into an even larger, more inclusive version of you, and that's your 5d self.
that's when multiple universes get introduced. each possible 4d self is a different universe in a way. but they are not real, they are possible. only the real life you are having is actual.
people who like the theory of multiple universes think that each possible 4d self is actualized, but in human life, we take the set of possibilities and extract one actuality from it. only 1 life is real. the others are potential.
that's how quantum physics gets involved. it describes, in multi-dimensional terms, the set of all possible outcomes. but humans don't experience all possible outcomes, just one. one actuality is extracted from the set of all possibilities.
i think it's wrong to thing that all possibilities are actualized, but some people go there because it['s one way to explain the weirdness of quantum physics. because quantum physics just describes the possibilities, and says nothing about how or why one actuality is extracted from the set of possibilities. but that doesn't mean they all exist, it just means we don't understand how the one that exists comes into being.
to me, it proves that there's more to human life than the physical world, more than what science can explain.
to put it more succinctly, quantum physics is a complete description of the physical world, but it is not a complete description of human experience, therefore, there is more to human experience than the physical world.