The numbers refer to the mass of each carbon atom.
Since the atoms are all Carbon, the number of protons in each atom is always the same ... if the number of protons changed, then you'd have an atom of something else, not Carbon. The only way the atomic mass can change and still be an atom of the same element is by adding or removing NEUTRONS.
Carbon-14 has two more neutrons in the nucleus of its atoms than Carbon-12 has.
