Number of protons and neutrons in an atom. Is approximate mass of the whole atom, since the mass of a proton and the mass of a neutron are both about 1 dalton. You can deduce the number of neutrons by subtracting atomic number from mass number.
Number of neutrons can vary in an element, but number of protons is constant.
The mass number is not exactly the same as an element's atomic weight, but we can use the terms interchangeably.