Quick answer
mV/A/m is the cable's voltage drop in millivolts for every ampere it carries over every metre of its run. It is the figure engineers use to check voltage drop without recalculating the conductor resistance each time. The drop for a circuit is the mV/A/m value multiplied by the design current and the route length, divided by 1000 to give the answer in volts. A smaller mV/A/m means a larger conductor and a lower loss. The value rises a little with conductor temperature and differs between single-phase and three-phase, so a datasheet quotes it for each installation condition rather than as a single number. It is read straight from the manufacturer's table, not derived by hand. APAR publishes a voltage-drop reference table for its LV cables.