What is relative molecular mass (RMM) and why use carbon-12?

This is the mass of the atoms in your compound relative to the mass of one atom of Carbon-12 (previously they used Hydrogen-1 or oxygen-16, but these have many naturally occurring isotopes). C-12 is easily measured and has less isotopes than naturally occurring oxygen and hydrogen, so was decided on as the internationally used convention to avoid any miscalculations based on the country research was being conducted in.
It is calculated by the average mass of one molecule divided by 1/12 the mass of a carbon-12 atom