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 , $37$, which is conveniently SUPERSCRIPTED.
Other chlorine nuclei may contain different numbers of neutrons; this is what gives rise to the existence of . The mass quoted on , $35.45$ $"amu"$, is the weighted average of the individual isotopes.
