Liouville's theorem is like a magic trick that tells you a few things about a special part of math called Hamiltonian. It's like a rulebook that tells us how things move around in space when there is energy involved.
Imagine you have a bouncing ball. When you bounce the ball, it has energy, and it moves around. But because of Liouville's theorem, we know that the ball can't just disappear or go faster and faster without any limits. The theorem tells us that the ball's movement will always be limited by the energy it has and the space it's in. This is like a fence that stops the ball from going too far or too fast.
Similarly, Liouville's theorem tells us that if we have a bunch of particles that are bouncing around, their movement is limited too. It tells us that no matter how many particles there are or how they're bouncing, they will always be limited in a particular way.
In summary, Liouville's theorem is like a set of rules that help us understand how things move around in space when there is energy involved. It ensures that the movement of particles is always limited and can't go beyond certain boundaries.