Mass number is simply the addition of the number of protons and neutrons in the most common isotope of an atom. Relative atomic mass is the mass of the atom relative to 1/12th of the mass of an atom of carbon. Average atomic mass is the average mass of all the isotopes of that atom multiplied by their frequency percentage (i.e. How often that atom is of this isotope out of a certain amount).
In case you don't know, isotopes are the same atom but with a different amount of neutrons, which slightly alters the mass of the particular atom. Isotopes often occur naturally, although in much lesser quantities. Hydrogen, for example, can be composed of one proton and one electron only, but there will occasionally be an atom of hydrogen with a neutron attached to the proton, creating the hydrogen-1 isotope.
Bit of linguistics: Note that an atom can only be an isotope to another isotope. For instance, "carbon-12" is not an isotope in itself, but it is an isotope OF carbon or an isotope TO "carbon-14". Isotopes are a link of comparison between two atoms of the same kind with different amounts of neutrons, just like "twins" is a link of comparison between two children. A single one of the two cannot be called a "twin": "they" are twins, one is twin to the other, or he has a twin (implicitly says "to him").
In case you don't know, isotopes are the same atom but with a different amount of neutrons, which slightly alters the mass of the particular atom. Isotopes often occur naturally, although in much lesser quantities. Hydrogen, for example, can be composed of one proton and one electron only, but there will occasionally be an atom of hydrogen with a neutron attached to the proton, creating the hydrogen-1 isotope.
Bit of linguistics: Note that an atom can only be an isotope to another isotope. For instance, "carbon-12" is not an isotope in itself, but it is an isotope OF carbon or an isotope TO "carbon-14". Isotopes are a link of comparison between two atoms of the same kind with different amounts of neutrons, just like "twins" is a link of comparison between two children. A single one of the two cannot be called a "twin": "they" are twins, one is twin to the other, or he has a twin (implicitly says "to him").