What is the difference between a subscript and a coefficient?

A subscript denotes the number of atoms bonded in the same molecule for instance O2 is diatomic and has 2 atoms in one molecule

A coefficient shows how many molecules are in a Compound for instance 2H2O shows 2 water molecules.

Subscript goes after the chemical symbol, and coefficients go before. Also, coefficients affect the entire chemical formula. Subscripts only affect one chemical.

2H O

2

There are 4 Hydrogen and 2 Oxygen.