The relative atomic mass is the combined mass of the total number of protons, neutrons and electrons. The mass of either a protron or neutron is 1 atomic mass unit (amu), whereas the mass of an electron is only 0.00055 amu, which is not usually counted. For example, carbon has 6 protons, 6 neutrons and 6 electrons. Adding these together, you have 6 amu for protons, 6 amu for neutrons and 6 x .00055 amu = .003 for the electrons. In total, carbon is about 12 amu.
A molar concentration is simply the total amu of the substance disolved in a liter of solvent. For example, a 1 M concentration of NaOH would be the total of the atomic masses of the individual atoms in 1 liter of water. The masees are as follows: Na = 23, O = 16 and H = 1. Therefore, 23+16+1=50. If you were to take 50 g of NaOH and dissolve it in 1 liter of water, you would have a 1 M solution.