Conservation of mass is a law or principle. It says matter cannot be created or destroyed. For example, take a fire. Wood fuels a fire. It may seem like the wood in a fire is being destroyed. But what actually is happening is a chemical reaction. The reaction is between the wood and oxygen in the air. The molecules in wood break down into atoms. Then, they connect to other atoms to form different molecules. That is why you see smoke from a fire. The atoms themselves are still there and don't change.
The mass, or amount, of the oxygen and wood before a fire would equal the mass of the smoke and ash left over after that fire. The mass of the materials you started with is conserved.
