Stepper motors us a cogged wheel and electro magnets to nudge the wheel round a 'step' at a time.
By energizing the coils in the right order, the motor is driven round. The number of steps that the stepper motor has in a 360 degree rotation is actually the number of teeth on the cog.
The motor we are using has 8 steps, but then the motor also incorporates a reduction gearbox of 1:64 that means that it needs 8 x 64 = 512 steps.
In this lesson, we do not use the common Red connection. This connection is only provided if you are using a different type of drive circuit that does not allow the current in each coil to be reversed. Having a center connection to each coil means that you can either energise the left or right side of the coil, and get the effect of reversing the current flow without having to use a circuit that can reverse the current.
Since we are using a L293D that is very good at reversing the current, we do not need this common connection, we can supply current in either direction to the whole of each of the coils.