Need good explanation of GRBL and to configure it !!!
grbl is the firmware/software that runs in the Arduino to interpret and execute G-code commands sent to the X-carve by programs like Easel, UGCS, and PicSender among others.
Is the grbl program already loaded into the Arduino when you buy the X-carve ??
Yes. Their current electronics package is the X-controller. It is an Arduino look alike (uses the ATmega328P processor).
There is a simple process that you can use to update grbl if necessary.