Step motor is to a machine to convert pulse to angle displacement. So stepper driver a certain pulse signal, it will drive step motor to a certain angle. you can control the angle the stepper moved by the number of the pulse. And you also can control the speed of the stepper rotate by the frequency of the pulse. Because the stepper motor consumes high current, MCU cannot control the motor directly. It requires a driver IC like ULN2003 to control the motor . The ULN2003 consists of an array of seven Darlington transistor pairs, each pair capable of driving a load of up to 500mA and 50V. Four of the seven pairs are used on this circuit.