In this circuit there is a Logic signal that is going to turn on and off the BJT (NPN) I assume 5V as logic value, the resistor of 1KΩ on the base of the BJT is a good value to limit the current to the base of the transistor since we need 0.7V VBE to turn on the transistor, in this case the resistor need to drop 4.3V, and we are going to have about 4mA passing through the resistor. When the transistor is switched on, all the current flows from the motor to the collector terminal and pass through the transistor, The Diode in parallel with the motor has the function to drop the flyback voltage emitted by the motor until it stops.
