The law of conservation of mass suggests that after a reaction, there is no change in the mass. It does not get bigger or smaller.
Using the chemical equation
For example, in the combustion reaction we have methane - CH4 and oxygen 02 which makes CO2 and H20
You need to balance this so you will end up getting
CH4 + 202 -----> CO2+2H20 (sorry I cant use the small formula numbers)
The mass of the products = mass of reactants which PROVES law conservation of mass :)