What is Post Processor

Have not received my OF yet, but learning Vcarve I did up a Project and wantef to save it but it tells me to i need the Post Processor, what is that

Not all CNC recognise all the G-codes. The post-processor is a component that generate the G-code appropriate to the specificities of your machine.
Conceptually it is similar to a printer driver that outputs the correct instructions for your specific brand of printer.

I use Fusion 360 so I cannot help with V-carve but 1F has posted instructions on how to select the post-processor: