relative vs absolute atomic mass
These are both indicators of the mass of an atom,
but relative atomic mass is an average, while absolute atomic mass is well
absolute.
Take Hydrogen, it has three stable isotopes
Hydrogen-1 AKA Regular Hydrogen, Hydrogen-2 AKA deuterium and Hydrogen-3 AKA tritium.
While a regular Hydrogen atom has an absolute mass of 1 Dalton
(which means one proton, Daltons is the amount of protons and neutrons combined,
you don't count electrons as their mass is 1800× smaller than a proton's)
The relative atomic mass of hydrogen is calculated by taking the absolute mass
of all hydrogen isotopes, each multiplied by their percentage of abundance
For example, Hydrogen-1 is about 99.96% of all hydrogen, Deuterium is about
0.04% and Tritium about 0.0044%
So we do 99.95 × 1 + 0.04 × 2 + 0.0044 × 3
to get to a relative atomic mass of 1.008