Collapsing the wave function is a term that scientists use to describe the phenomena where waves of energy collapse to form a particle when they are observed or measured in an experiment. “Collapse” is the word that science uses to explain this theory, but another way to describe it would be to say that waves of energy “appear” as a particle when they are observed or measured.
Essentially science tells us that all of reality starts out as a wave of energy and that wave, no matter what it is in our current reality, is all possibilities of that wave until it is observed. Sounds kind of familiar, right? Basically all of existence, every multiple universe, is true and present until we settle on the one we are in by, well, observing it. Schrodinger's cat is the most famous thought experiment of this concept.
This is the radio analogy that we use throughout this site: Infinite "stations" are present but we aren't sure what is playing until we tune into one, we just happen to only be aware of our current station.
In 1927, Werner Heisenberg used the idea of wave function reduction to explain quantum measurement. Nevertheless, it was debated, for if collapse were a fundamental physical phenomenon, rather than just the epiphenomenon of some other process, it would mean nature was fundamentally stochastic, i.e. nondeterministic, an undesirable property for a theory.