The law of conservation of mass says that matter cannot be created or destroyed but only changed from one form to another. In a chemical reaction this holds true. When atoms undergo a chemical reaction they may change form or bond with other atoms. However, there should be the same amount of atoms of an element at the beginning and at the end of a reaction. For example, when sodium and chlorine undergo a chemical reaction the equation looks like this:
There is only one atom of sodium as a reactant and one atom of sodium as a product.
The same is true for chlorine.
P4 + 5O2 --> 2P2O5
Although phosphorous and oxygen are different in the products and reactants because of the chemical reaction they have undergone, there is still the same amount of atoms of each on either side of the reaction.