The law of conservation of mass states that the mass of an isolated system will always remain constant, regardless of the processes acting inside the system. The matter cannot be created or destroyed, it only changes form. Basically this means that in a closed system : REACTANTS MASSES = PRODUCTS MASSES. This law is non-relativistic - the true (relativistic) situation is somewhat more complicated. The law was first formulated by Antoine Lavoisier, who is often referred to as the father of modern chemistry.
This law finds application as an approximation in cases where relativistic corrections are small - for example in chemistry.