The subscript in a chemical formula refers to the number of atoms of each element present in a molecule or formula unit.

Here's a breakdown:

* H₂O: This formula represents a water molecule. The subscript "2" after the "H" indicates that there are two hydrogen atoms in each water molecule.

* CO₂: This formula represents a carbon dioxide molecule. The subscript "2" after the "O" indicates that there are two oxygen atoms in each carbon dioxide molecule.

* NaCl: This formula represents a sodium chloride (table salt) formula unit. There are no subscripts because there's one sodium atom and one chlorine atom in each formula unit.

Important Note: The subscript applies only to the element directly preceding it. If there's no subscript, it's understood to be "1".