The Law of Conservation of Mass states that matter cannot be created or destroyed in an isolated system. In other words, the total mass before a chemical reaction is equal to the total mass after the reaction.
A system where both energy and matter can transfer between the system and its surroundings - this would be if you left your door open while rearranging your furniture.
A theoretical concept where neither energy nor matter can enter or exit – imagine if your room was sealed off from the rest of universe while you were moving around your furniture!