The power supply is indicated for the supply small 6 V motors with current up to 500 mA. The resistor R1 can have values between 2.2 ohm and 15 ohm depending on the motor. The circuit can also be used with 12 V motors using a transformer with this voltage and changing R1 to 22 ohms x 2 W. The circuit is from the book Fun Projects for The Experimenter by Newton C. Braga.
