When he means "runs in the background of mach" it means the custom VB code written and saved as macropump.m1s is being executed repeatedly, say 20-25 times per second, depending on the complexity of the code.
The macro will start running when Mach3 is loaded, and will stop running until you exit Mach3.
I'm not quite sure if macropump will still be running if the E-Stop is pressed, can someone clarify this?
Its best to keep the code as simple as possible, in order for Mach3 to be more versatile.
Just my 0.000000 cents XD