Try changing the command that sets the speed of the stepper motor:
to a lower value (say 5) upload the sketch and notice that the stepper turns more slowly.
Now try and find the maximum speed for the stepper by increasing the speed above 20. After a certain point, you will find that the motor does not move at all. This is because it just cannot keep up with the stream of pulses asking it to step.
Try disconnecting the orange and pink leads of the stepper. It should still turn, but you will notice that it is weaker, as it does not have both coils working to push the motor around.
In the next Lesson you will learn how to make an Arduino detect movement using a PIR sensor and then communicate with your computer to send an email.