Rephrase the law of conservation of mass in your own words

the mass of matter remains the same even when undergoing a chemical change because matter can neither be created or destroyed

The law of conservation of mass basically says that nothig can be "created" or "destroyed" but that thigs just under go transformations and change into something else. For example, take a pile of wood. If light it on fire it doesnt disappear, i have just changed it into ash and thermal energy.