Can someone explain this whole theory of relativity and what just happened yesterday?

I think you're getting a little lost in the metaphor. Quantum physics deals in quantities called observables, which are Hermitian operators that act on the state vectors of the system. The possible observed outcomes of an experiment are the eigenvalues of the observables. When you conduct an experiment you will observe one of the possible outcomes. You can't observe a system to be in superposition; that makes no physical sense, for one, and for the other, mathematically, that's not one of the eigenvalues of the observable operator.

The question that Everett tried to answer was what actually happens to the system when the observation is made? Does something occur that collapses the system such that (for example) the particle passes through only one slit, while when the apparatus is set up differently the particle passes through both slits? Everett's interpretation says that rather than thinking of it in terms of collapsing the system, you can equivalently think of it as coupling the apparatus to the observer, so the observer becomes part of a larger system. In other words, rather than there being a small quantum system with eigenstates "the particle passes through the left slit" and "the particle passes through the right slit" there's a larger system with eigenstates "the scientist observes the particle passing through the left slit" and "the scientist observes the particle passing through the right slit."

At the largest scale, the entire universe could be represented by a single vast state vector, and the state of the universe could be described by a time-dependent Schrödinger equation. Of course, that's not a useful thing to thing about because no such equation could ever be written down, but Everett's point was that in principle such an equation could exist, and if it did exist then there would be no need to think in terms of wavefunction collapse at all.

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