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.
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous thread is at close to 500 comments.