When we write the formula of a compound, we use subscripts to describe how many atoms of each element are present in one molecules of the compound. For instance, the formula for water is H2O. This tells us that each water molecule is made up of two Hydrogen atoms and one Oxygen atom. By contrast, H2O2 is the formula for hydrogen peroxide, which has two Hydrogens and two Oxygens per atom.
The law of definite proportions is the chemistry principle which discusses the fact that every compound has a fixed formula, with its component elements always occurring in the same ratio to one another.