For a huge oversimplification:

The cosmos is a big list of world-states, of the form "electrons in positions [(12.3, -2.8, 1.0), (0.5, 7.9, 6.1), ...] and speeds [...] protons in positions...". To each state, a quantum amplitude is assigned.

The laws of physics describes how the quantum amplitude shifts between world states as time goes by (based on speed of particles and various basic interactions ....).

Conservation of matter says that for each world state, you can compute the amount of matter (and energy) inside, and it stays the same.