19: Potassium (K) is the ordinal number of the element. It indicates how many electrons or protons there are in 1 Potassium (K) atom, i.e. there are 19 electrons and 19 protons.
39.098: is the relative atomic mass of potassium. It represents the mass of 1mol (6.02 * 1023 atoms) of Potassium. it also means how many times greater than 1/12 of the weight of the C12 atom