According to the law of conservation of mass, the quantity of the elements, involved in chemical reactions does not change. For example,
H2O2 - > H2O + O2
is wrong, because there are two O atoms on the first side of the equation, and three on the other. To correct it, coefficients must be added, until the amount of both H and O atoms is equal on both sides.
2H2O2 - > 2H2O + O2