The law of conservation of mass says that matter cannot be created or destroyed. This means that when a chemical reaction happens, you can never create new atoms or destroy the ones that are reacting, they simply rearrange themselves into new substances.
For purposes of looking at chemical equations, this means that the equation that describes the reaction has to be "balanced". In other words, there must be the same number and same kind of atoms on the reactant side of the equation as there are on the product side to ensure that no atoms are created or lost during the reaction.
