The law of conservation of mass says that no mass is lost or gained in a chemical reaction. Atoms are not made or destroyed. They are rearranged into new substances. So the total mass of the reactants is equal to the total mass of the products.
This is why chemical equations must be balanced. A balanced equation has the same number of atoms of each element on both sides, so the mass on the two sides must match. For example, in 2Mg + O2 → 2MgO there are two magnesium atoms and two oxygen atoms on each side.
You can use the law to find a missing mass. When 24 g of magnesium reacts with 16 g of oxygen, the product is 24 + 16 = 40 g of magnesium oxide. When 100 g of calcium carbonate is heated, it forms 56 g of calcium oxide and some carbon dioxide gas. The carbon dioxide must have a mass of 100 − 56 = 44 g. The mass of the reactants and the mass of the products always agree when every substance is counted.