Stepper motors respond not as smooth turning armatures as normal electric motors do but rather they turn in many small choppy but even angular increments per revolution.
If you do it fast enough, the motor seems to rotate smoothly but it is actually "cogging" as it rotates as each step pulse is applied. Each "step" pulse to the motor driver will cause the motor to increment 1 angular increment.
"Direction" signal provides exactly what it sounds like.
The motor can turn CW or CCW. A HIGH "direction" signal will cause the motor to rotate CW (or vice versa) whereas a LOW signal will cause the motor to rotate in the OPPOSITE direction.
Hence, the "step" and "direction" aspect of stepper motor operation.
The above is a very simple explanation of how a stepper motor works. You need to do some research into stepper motors to develop a better underestanding.
I know how stepper motor works , all that i want to understand is how the cnc software understand the 3d design dimentions and give signals to the parallel port for each of the three motors , i want to know how it works or to undrestand the algorism