Junec, a potted motor is a motor mounted behind the arm (the wheel end) of the machine. It runs the belt that moves the needle bar. Many of the old treadles were models that were made later and electrified by this means of adding a motor on to turn the belt of the wheel. And lights were added sometimes through this box to shine on the needle plate from the back of the machine.
A "potted" motor is extra to the machine and is therefore easier to remove. You can make a hand crank or go back to a treadle easily if it is a potted motor.