It tells you how many and what elements this compound containseg.water's formula is H2O- this shows that it contains 2 hydrogen atoms and 1 oxygen atom.
The chemical composition of the compound
Subscripts give you the number of molecules in that compound. (For example, H2O contains one molecule of Hydrogen.) Superscripts give you a charge.
Molecular formula of a chemical substance is representative for the composition of this chemical substance.
the numbers are what is in a compound number