Its a unit of charge and not capacity. You have the full charge voltage, and the cutoff voltage where you assume the battery be drained (min operational voltag

Its a unit of charge and not capacity. You have the full charge voltage, and the cutoff voltage where you assume the battery be drained (min operational voltage for your circuit), and the charge that gets you from the first to the latter is the mAh number. I find it more practical than using plain energy (Wh or Joule).