chemical formulas show what atoms are in a molecule. In this case there is 1 hydrogen (H), 1 chlorine (Cl), and 3 oxygens (O). The 3 behind the oxygen is a subscript and tells us that there are 3 oxygen atoms.
Another example is H2O which as 3 atoms. 2 hydrogens (H) and 1 oxygen (O). This formula has a subscript 2 behind the hydrogen showing that there are 2 hydrogens.
