In a chemical compound, the masses of the elements are always in the same (i.e. fixed) proportion (ratio).
Basically, same definition as a compound.
Definition of Law of Multiple Proportion
Whenever the same elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in a ratio of small whole numbers.
What does it means, You can't have a fraction of an atom.
Definition of Law of Conservation of Mass - Already in your notes