Koopman-von-Neumann Mechanics is a reformulation of classical mechanics using the same language that we usually only use in quantum mechanics.
Usually in classical mechanics, our main focus is the trajectories of objects.
In contrast, in quantum mechanics, our main focus are observables, states and measurements. This means, instead of describing the trajectories of objects, we now ask instead for example: If we measure the momentum of this ball, what's the probability that the result will be $10$ kg m/s?
The equation of motion in Koopmann-von-Neumann mechanics is a complexified reformulation of the classical Liouville equation (which handles probability distributions over the phase space).
The Koopman-von-Neumann reformulation of classical mechanics is extremely helpful to understand the differences between classical and quantum mechanics. Usually these theories are described in completely different languages which makes it hard to compare them. Using the Koopman-von-Neumann reformulation we describe classical mechanics in exactly the same language that we use in quantum mechanics and thus can perfectly analyze what is different.
Ordinary mechanics must also be statistically formulated: the determinism of classical physics turns out to be an illusion, it is an idol, not an ideal in scientific research. Max Born, 1954 Nobel Prize Lecture.