Why is the naming meaning of Compound DC Motors?
I want to ask, why is the Cumulative Compound DC Motor called
  Cumulative and why is the Differential called Differential?


A compound DC motor has two field windings. These are called: -


The series winding
The shunt winding


The effects of one winding can be cancelled (or enhanced) by the other: -



Image taken from this slide player.

You can get various torque and speed versus loading effects by operating differentially or cumulatively: -



Picture source