Coefficients in the reaction show numbers of moles in with substances are react or form.
2Kl(aq)+Cl2(g) ----> 2KCl (aq) + l2(g)
2 mol 1 mol 2 mol 1 mol
If we look at the reaction we can see that from 2 mol KI and 1 mol Cl2, form 2 mol KCl and 1 mol I2.