Simple Definition:

Mass Number is the total number of protons [positively charged particles] and neutrons [neutral particles] in an atom’s nucleus [core]. For example, the mass number of carbon-12 [a type of carbon atom] is 12 because it has 6 protons and 6 neutrons.

Very Simple Definition:

Mass Number is the total number of protons [positively charged particles] and neutrons [neutral particles] in an atom’s nucleus [core]. For example, carbon-12 [a type of carbon atom] has 6 protons and 6 neutrons, so its mass number is 12.