What does the atomic mass tell you?
The combined amount of protons and neutrons there are in an atom.
That's it. You might notice, however, that I did not mention electrons. This is because their mass is so small in comparison to that of protons and neutrons that it is negligible (can be ignored).
You might also notice that some elements have decimals in their relative atomic masses. This is not because the have half a neutron/proton (:D) but because they have isotopes (the same element, with the same charge) with different amount of neutrons (notice that a neutron doesn't have charge, so more neutrons only means more mass).
Hope it Helps! :D .