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 - Dalton (1766-1844)
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 - (Lavoisier - 1743-1794) - Already in your notes