We know that EVERY Cl contains 17 protons, massive, positively charged nuclear particles. This is what defines it as a chlorine atom (and in fact, given the atomic symbol Cl, the subscript is a bit superfluous). However, the nucleus can also contain NEUTRONS, massive, neutrally charged nuclear particles. Interactions between neutrons and protons help stabilize the nucleus (and I am not a particle physicist). The sum of the massive particles, 17+20, gives the mass number, 37, which is conveniently SUPERSCRIPTED.
Other chlorine nuclei may contain different numbers of neutrons; this is what gives rise to the existence of isotopes. The atomic mass mass quoted on the Periodic Table, 35.45 "amu", is the weighted average of the individual isotopes.
