The molecular formula indicates the number of atoms of each element in the molecule.
Oxygen exists as molecules (diatomic molecules). The fundamental particles that make up a sample of Oxygen are pairs of atoms, rather than individual atoms.
You can count the number of atoms of each element in a chemical formula by multiplying the coefficient by the subscript, recognizing that those coefficients and subscripts that are not shown are equal to 1. So, in our case there are 6 atoms of oxygen.
