what information is provided by the atomic mass?

Answer 1

The atomic mass minus the atomic number = the number of neutrons.
Let's take Sodium

Mass - number = number of neutrons
23 - 11 = 12
The atomic mass is also used in mole calculation of substances.

Answer 2

I'm not sure I buy the explanation by Proch. Proch's explanation correct for "mass number" but not atomic mass. The atomic mass is the weighted average of the masses of all of the isotopes of an element.

Answer 3

True.
Sorry I'm french, so I kinda did a translation from all I learned in french. I mixed up mass number and atomic mass.