G_Code is the generic name for a control language for CNC machines.
It is a way for you to tell the machine to move to various points at a desired speed, control the spindle speed, turn on and off various coolants, and all sorts of other things. It is fairly standard, and is a useful tool.
The Start/End G_code are the startup and end procedure of the print. These pieces of code influence the startup procedure and can be customized.
By default the printer is heated up, homed to the corner, and the head is primed. Editing this requires knowledge of G_Code.