Quantum Mechanics employs Wave Functions, which are mathematical tools for expressing the state of a system.
The main issue with the wave function, among physicists, is does the wave function actually exist, or is it just a symbolic representation of something that exists?
More specifically, wave functions compute the probability that an event will be observed.
The position that the wave function exists in and of itself, is the psi-ontic perspective, whereas the idea that it describes something existent, is the psi-epistemic perspective.
However, once an event is observed, the wave function has to be updated, because an observation being made rules out all other probable observations, meaning the probability of the measured observation must now be one.