A Post Processor is a unique "driver" specific to a CNC machine, robot or mechanism; some machines start at different locations or require extra movement between each operation, the Post-Processor works with the CAM software or off-line programming software to make sure the G-Code output or program is correct for a specific machine build.
The Post Processor controls the format and syntax of the program that is generated for the controller that controls a specific machine (typically a CNC or a robot).
A post processor is used with the CAM (Computer Aided Manufacturing) Software to generate G-code for a specific CNC Machine. A post processor can also be used with Off-line programming software for a specific industrial robot.
The Post-Processor will alter the program output to suit a specific machine; a "Post" can be used for complex things like producing a proprietary machine language other than G-Code or M-Code, or a Post-Processor maybe used to start a machine from a specific position.
Another example of use for a Post-Processor would be an ATC (Automatic-Tool-Change) for a CNC, the Post-Processor is required so the correct Tool is collected from the correct location.
Some devices connect to the computer using "Serial Communication" and some CNC devices connect using "Parallel Communication", the Post-Processor does not influence the "communication", the Machine Software does.